The Book is interesting and fun.
The movie is a joke.
The book spends the better part of the book building up who Percy's father is. It is no secret he's a demi god, but it spends a long time trying to make the reader guess.
The movie reveals it in the first 30 seconds.
The movie goes from scene to scene with adequate but mostly forgettable flare after that. Its not really good, but its not horrifically bad either; about on par with the average forgettable fantasy movie until they get to the scene with the minotaur right before they get to the camp.
First of all, this is a critical scene that defines Percy's character, and they do it in about 2 minutes 30 seconds....but even that is forgivable (if not a tragic loss.)
Then his mother dies.
He looks about as upset as if stepped in a pile of dogsh*t. He doesn't cry. Doesn't stoically swear revenge. He just goes, (essentially) 'bummer'. Then they proceed to show him the camp.
And that's when I stopped watching.