The relationship between Pierce and John is interesting. Although Pierce got on my nerves a bit in this book at times, she's an overall pretty likeable character. I admire her determination and commitment to those she loves. Like all of Meg Cabot's books that I've read so far, the characters' actions and voice seem appropriate for their age. Pierce is not a 17-year-old girl acting 30. She makes stupid choices and whines just like any teenager would.
I think the best part of Underworld, though, has to be the subplot. John's history is dark and helps move the story along, while Alex's delimma really lights sparks. I found what was happening with Alex to be biggest driving force so far. I will even admit that the surprise at the end caught me off guard. (Not so much what happened, but who was there to witness it.)
Like any good middle book in a series, Underworld ends with a cliffhanger! So, if you don't like cliffhangers, you better wait to read this one until the final book comes out. You will be happy to know, also, that this is a super fast read. I was able to finish this one in just a few hours. Hooray for summer reading!