I finally finished reading the first Percy Jackson book and saw the film today as well.
The book is written really well. Rick Riordans style of writing really emotes twelve year old Percy's rebellious attitude as a kid who has trouble fitting in and following the rules. I really liked the first half of the book where Percy discovers that he is a Demigod. The book got a bit monotonous once Percy embarks on his quest. The quest basically had Percy face one Greek mythological character after the other on his travels. I really didn't enjoy the quest part of the book. The book ended pretty well though, however I had guessed who the villain was half way through. The end was predictable but nice. Even though the book has some flaws, by the end of the book I did find myself wanting to journey with the characters again, so I guess I will read books two to five as well.
Now for the movie. Director Christopher Columbus changes the sequence of events and some of the major plot points. In fact he goes so far as to change one of two villains (no spoilers ahead, I won't spoil either the book or the movie for you.) and some of the locales where battles in the book take place. He even changes the sequence of events as to how Percy obtains a magical talisman that helps him escape the underworld! Die hard fans of the book would be disappointed but the movie is a lot of fun to watch as a whole. Frankly I preferred it to the Harry Potter movies. The kids playing the main characters in the Harry Potter movies are frankly quite stiff and painful to watch.