Having already read the book Storm Glass, I'm able to compare the two books. The plot and character progression are exactly the same. The protagonists have bad starting points with something they both excelled at as kids: for Yelena it's arcobatics and for Opal it's glass making. While Opal already knows she has magic, they both have to find out the extent of their powers. The similarities do not end there. With each having a shadowed past, they are originally unconfident and gain more and more confidence as the novel progresses. They both slowly fall in love (something that anyone who reads should know, so its not an exactly surprising piece of information), and proclaim how they feel by the end of the novel. Even their lovers have similarities: both are powerful (in their own individual ways) and don't show much inclination to really liking the main character until the end of the novel. 
 Don't let these similarities cloud your judgement; I certainly didn't. The book is a little more violent than Storm Glass and I found Yelena more sarcastic. One thing that I found interesting, was having read the first book of the second series in the same world, I was able to see more differences in Yelena's personality and enjoyed watching her slowly progress toward that point. 
 CHARACTERS: 
 - Yelena (last name not supplied until last chapter so thus not to be mentioned): Originally a bit of drag (though for a good reason), made me sympathetic toward her, and I wanted her to get better. Humorous, fiercely determined and loyal. Also has a soft heart under all the sarcasm. 
 -Vale (was his last name even mentioned?): Even though he seems detached at times, I actually enjoyed the development of his character. Well, at least the development of him in Yelena's eyes. He is supremely loyal (the "I will actually die for you if it seems the best way to help" type), extremely sexy from all his kick-ass awesome skills, and actually quite compassionate. 
 -Janco (seriously what's with the lack of last names?): Having read Storm Glass, I already had an impression of him, one of good cheer and loyalty. However, that was not my first impression in this one. I first thought he was a dek. His true character shone through eventually and I basically fell in love with him. The little tid bit you get on him intrigued me. 
 -Ari (whose full name is odd and only mentioned once, and obviously he doesn't have a last name): He seems to be the laughing one in the beginning, but as I read I found him to be more serious (seriouser?) but with a dog level of loyalty in a totally unannoying way. He's a good character. 
 -Commander Ambrose (or is it? Hehe): Cold, scary, and indifferent were my first impressions. However, he becomes very human, and very commendable as I read more and more (considering all the bad advertisement I had gained from Storm Glass about him that's saying something). I truly liked his character and the amount of depth that Snyder put into him. 
 That should be enough about the characters, as the bad characters need to be read in full context. There shouldn't be any preconceived notions about them. 
 And the end.