Book 2 carries on the tradition from book one, so if you liked book one (I did, with reservations), it's the ongoing story, very similar tone, but slightly darker. It's no longer "coming of age" though... everyone grew up real fast!
There are some nice "surprises" although I guessed some of them. There's more sex to the point it's repetitive and tiring -- I kind of skipped those scenes. The sex scenes don't bother -- they just feel totally out of place, and it made it impossible to recommend to any YA audience who might otherwise have been interested in a good (although violent) adventure. 
Don't get me wrong, I still enjoyed, and I'll be reading the third book (in January, I think?) but I wouldn't necessarily recommend to too many friends, and definitely not to my children. That sounds a little contradictory, doesn't it. I guess that's how I felt at the end, conflicted. Oh wel.
The other thing that became very noticeable is the excessive love of military, to the point where it's almost the main theme. (The military theme was there in book one, but it's more noticeable in the second book, which is very much a war novel.)
I don't mind reading all three genres separately -- fantasy with dragons, romantasy with sex, and well drawn military drama -- but together, here, it doesn't mesh totally. 
But still, I'll be in line to read the third book in 2025. 

Yarros does such a great job in building a world where everything makes sense. She'll leave hints along the way that resolve later on in ways you don't expect. I love having the referenced materials starting chapters (also happens in the first book) as it's a smart way to contribute to the world building and give the reader additional context or even a clue of what might come in the upcoming chapter.

I started this book right after the first one and I was really hiped, but I think that  some things could had been handled better. First of all some scenes could be shorter and still reassume everything perfectly, the FL is too dumb, which is really strange because in the first book she was the smartest ( now I understand the plot exigence but that is too much), Xaden is the most coherent character ( he makes a choice and just goes with it),  the side characters are too marginal and poorly developed. The author, in my opinion, dedicated too much space to Xaden and Violet's arguement.
Last but not least Tairn and Andarna best characters, change my mind.

P.s. it might seem that I didn't like the book but I actually did, in my opinion somethings could have been written better.

This book could have and should have been either half as long or two separate books. There is so much that happens that it’s hard to encapsulate how I feel without the end-of-the-book bias. 

In general, reading this series is particularly difficult because our narrator Violet is a mildly illogical, whiny, savior complex of a protagonist whose companion is a brooding, clever, intimidating man who has a savior complex of his own. This makes for redundant conversations about them keeping secrets from each other (despite being at war and of different ranks) and if they actually love each other (which quite frankly gets so elementary in the back and forth). I hate that Yarros adds this powerful female character Cat and makes her entire storyline essentially about jealousy and a man. And even though Cat herself insists it’s “not about a man,” there are taunts that go on for chapters about this quarrel. It’s frustrating that this is how Cat’s persona is basically defined from the get-go when she actually had potential to be more.

You can also tell this is a fantasy genre book because the names of all of these people, dragons, and griffins are hard to remember and pronounce. There was a mass influx of new characters and combined with the recurring characters, I couldn’t keep track. I think side characters like General Sorrengail and Imogen and Dain had better character development. And again, we are limited from Violet’s thoughts and point-of-view. But boy, did I need a break from her sometimes. 

This is an incredibly plot-driven sequel that takes the characters through many tribulations such as: joining a revolution, political bargaining, dealing with previously-thought dead siblings, the aforementioned love story drama, torture and interrogation, schemes to break in to places, and more. Again, I feel like this could have broken up into two books or at least been shorter. There was almost too much setup about merging the fliers and riders and saving Basgiath. While every chapter ends on that cliffhanger-type way, I wasn’t excited for what was next until maybe halfway through. 

I will say that I was pleasantly surprised with the handling of Mr Varrish. I’m interested in seeing where Aaric’s story goes. 

Overall, it was an entertaining read with a couple unexpected twists. I got through it quickly. But the writing and lack of true character development hampered this for me. 

The terribleness of this book was everything I was expecting but didn't get in Fourth Wing. The dragons are carrying more than just their riders in this series and their wings might be crumbling under the weight.

This was so obviously rushed. The ebook version I checked out wasn't immune to the errors left behind by the rushed publication - I've marked several points where spacing was missing, I can't tell if all the sentences that were indented to a new paragraph mid-sentence were intentional or not, and there was even one new line that wasn't indented at all. But the worst of it was the story, which clearly needed more time to be refined and more feedback to help that refinement. Bad decisions with worldbuilding didn't just start in this book, but man did it start tearing at the seams.

Even the stuff that I thought was fine in the last book, it's fallen off in this one (thankfully not the dragons, unless I'm just too biased). The romance went downhill with repetitive drama

that combined with the other side of the love triangle's redemption arc this book

I genuinely can't tell if they're even end-game anymore, it's just gotten so frustrating to read and I can't even believe Violet could be in love with Xaden anymore. I was fully expecting him to be Tamlin'd

and Dain to become the non-shadow daddy Rhysand

, and I'm still lowkey waiting for it to happen next book. Honestly lowkey ship Dain and Violet more than I do the supposed end-game couple.

But don't worry, it wasn't just these two that started to get on my nerves. Each half of the book has one character so insufferably and stupidly evil that it's genuinely a shock they weren't shot on sight by the rest of the cast, they are literally too much! In the first half it's Varrish, who is a prime example that the author doesn't understand how corrupt systems maintain themselves and is just a worse echo of better revolution stories, because even egotistical dictators aren't this blatantly sadistic without at least securing all sectors of power first to avoid consequences.

He should've been ousted from the story the moment his dragon torched a batch of students for no reason, there is no way any of the other faculty should've let him get away with that - if Colonel Aetos getting punished is meant to show there's at least some standards within the corrupt military, and clearly the rest of the staff are upset with Varrish's actions, why does it take nearly 300 pages for someone to finally kill him in a culture where offing vague "liabilities to the wing" is encouraged and why is not any of his peers that despise him the does it?! Oh, and speaking of his dragon, if dragonkind protects dragonkind why is Solas not doing anything about Varrish's blatant harmful intent towards a juvenile?

As for the second half, the little goodwill I had with last book not making Imogen a spiteful love rival for Xaden's affections was thrown out when Catriona got introduced. Her unhingedness sometimes rivals Varrish's and again I'm wondering why the fuck even her own people isn't hurling her off a cliff.

She cheated in a spar against Violet in front of everyone, her own people were horrified by the lengths she'd go to ruin Violet's life in that match, and yet she inexplicably is still paired up in the same team as Violet?

No, she inexplicably is still allowed to be part of the army when she's a clear fucking liability to the cohesiveness of the army and the health of their already tenuous alliance? For a series that likes to throw around the word "liability" so much, it has a hard time fucking recognizing a clear example of it!

And the balls to make her say "you think I'd fight over a man?!" after a whole fancy ball scene where all her dialogue paints her as the classic "pretty mean girl is upset the love interest chose the not-like-other-girls girl over her".

I can't even appreciate the times where the story tried to humanize and soften her up a bit, because she was just written to be so unreal of a human being. And tangentially related to her is how many things happen in the second half just for the sake of adding more conflict without any care for making it all meld together.

Twice does leadership put on challenges to try and build comradery amongst the riders and fliers, and both times all it is is an excuse to create more conflict, with future conflict acting immune to the original goal of these challenges, and then somehow after all the bitter and petty hate in spite of working together in life-threatening situations, suddenly at the climax of the book the fliers are ride or die for the riders?

Characters are not written to be people in this book. Nobody acts like a real person in this book.

I truly believe that besides giving more time to writing and revising this book before release, it also should've been split up into two separate books. It already feels like two books Frankentein'd into a bundled edition. I think it would've given more time for the relationship drama to breath (which is a start in fixing the mess that it was) and genuinely it just feels like the more logical and fitting option. I even took a break in between "Part 1" and "Part 2" (even the book recognizes these are two distinct plots) because of my first library loan expiring and it felt less like returning to Book 2 of the series and more like picking up Book 3. It just doesn't make sense to me why it was combined; even if there was the concern about some plotlines not being resolved in the first half, it's normal for some of them to carry over between installments.

Overall, this was rushed. More time was needed to think through the plot and the worldbuilding. I think regardless of whether you liked the first book or not, this one is clearly inferior. Despite all the anger I have for this one, I do hope the fact Onyx Storm took a whole year is a sign that both the author and the publisher learned their lessons and Onyx Storm will be as okay or even better than Fourth Wing. And if the downward trend of quality continues, I at least hope the characterization of the dragons isn't the next thing on the chopping block. They are legit the only reason I don't mark "No" for "Are there loveable characters?"

Additional edit: Also so much of the venin lore introduced is stupid.

Jack was a venin since his first spar with Violet - HOW, since when would he know what a venin is, that he could become one, and how to become one, at least give us a throw away line! Venin have been hiding out amongst the citizenry this whole time - so fucking cliched and unnecessary, although if they retroactively reveal Varrish as a venin and some of the stupid-corruption of the military on their infiltration I'll at least buy that. Xaden becomes a venin - literally why is it so easy and why, if it's so easy, is this the first time we're seeing a character tempted to the dark side?!

This series is my Roman Empire.

I related literally everything I could to it, turning my personal reality into its universe. The heat wave Joburg experienced? We had dragons breathing down our necks, or they were trying to break us in land nav 😂

The lightning storms we had were perfect timing for this series 😍 especially for the last battle - reading it during a storm was everything ⚡️ 

This book goes into politics, truths, and rebellions a bit more. Still made me cry. Violet is even more badass. I really loved that she didn’t become someone who couldn’t handle her trauma. 

Read if you love:
-dragons
-shadow daddy’s
-magic
-banter with friends
-dropping all your money on merch

Not as good as the first book, and the ending wasn't as shocking to me as everyone said it would be. I thought what happened at the end was going to happen to Violet, not the other person, but still. I have a feeling Onyx Storm will annoy me now, and I'm not as excited about it.