I really liked this book, and definitely loved the characters. I also love the greek mythology aspect of it, as it is the first modern story that I have read with it so well incorporated.
Unfortunately, I could tell exactly what was going to happen from about page 100, so that was kind of a letdown. Also, a lot of the plot twists and turns, and even the main layout, appear to be VERY SIMILAR to Harry Potter, so it's not totally fresh. The good news though? I LOVED Harry Potter and so this was definitely a good read, especially since I feel that we know more about what Percy is thinking than we ever did with Harry (seeing as how this book is written in first person).
All in all though, I would definitely recommend this book to anyone who likes greek mythology, or anyone who doesn't even know a thing about it (a.k.a. me). Also, the chapter titles rock!