This was OK overall I liked it (a bit like junk food is good but not filling.)  The concept was interesting and the smut was OK.  But I found Rae really annoying think slasher film victim.  She made all the worst decisions and had absolutely no self-preservation.  I found Leon more interesting with the author exploring male bisexuality and his long term friendship and love of his fellow demon, this added a more unique twist.  But while the chemistry was there for the smut, there was no on-page development or work done to make them fall in love.  And ultimately this is supposed to be a love story, not just smut.  I was never convinced they even liked each other never mind falling in love.
I think for the length of this book the love story and the characters lacked any development.  I was never shown/convinced of Leon's power or history really so that was another missed opportunity. 
Overall it was fine, the biggest strength is the atmosphere, the smut (a lot of trigger warnings in the book but overall done responsibly I think) and the 3 main demon characters, but won't be a re-read nor will I continue in the series.