I enjoyed Sky on Fire, but not as much as the first one. Sky on Fire has a much darker tone than Monument 14. I'm ok with that, it just took me by surprise. There are quite a few unpleasant moments in Sky on Fire. It's not really graphic, but descriptive enough to make my skin crawl a bit. There is also a scene in the first book that is similarly repeated, not once, but twice in Sky on Fire and I could have done without that. 
 There are quite a few characters to keep track of, but it's a bit easier in Sky on Fire because they are split into two groups, those that stayed behind and those on the bus. Dean was still my favorite. Even though he did some crazy things he showed remorse for his actions, even though he wasn't necessarily in control of himself at the time. Astrid changed for the better in Sky on Fire, but still maintained her awesome level of sneakiness. There are a couple of new characters introduced, and it should come as no surprise that most of them are bad. 
 Dean, Astrid and the kids that stayed in the Greenway have to deal with people trying to get in while Alex, Niko and the rest of the kids took the bus out to save Brayden. The kids that left had it way worse. You wouldn't think so but the outside was crazy for real. I'm messed up enough to say that I liked the crazy on the outside better than the crazy on the inside. 
 I had two problems with the ending. For one there's a bit of a happy ever after moment that weirded me out. The second was that the ending did not make me crave the third book. In fact if you would have taken out the last couple of paragraphs the series could have ended right there and I would have been completely happy. Sky on Fire also felt a bit rushed, I wish it was a bit meatier, but if you're looking for a quick read Sky on Fire is a good choice. 
 Overal if you like crazy, messed up, post apocalyptic books Monument 14 and Sky on Fire are for you.