Overall I liked the movie, but it didn't really follow the book's plot. The book went a lot deeper into things, and had much more humor.
[BOOK AND MOVIE SPOILER ALERT]
I don't know if anybody's said this yet, but the whole thing.... if anybody's read the book, in the book, Ares is the lightning thief, yet in the movie, Ares isn't there at all. .-. Luke was the Lightning Thief, which really screwed things up.
[/spoiler]