It sucks how people seem to constantly compare this to the book. Sure, the movie itself is based on it, but I feel that less people would have complained about it if it didn't have Percy Jackson in the title. Nobody said the movie had to be the exact same as the books. Anyone who does is just being childish. A perfect movie adaptation has never existed, and it never will. Personally, I think I enjoyed the movie's story more than the book - but you know, that's just me.
As for the movie itself, it's a dumb movie. That's just what it is. It's a fun action adventure, and it does what it tries to do. It can be genuinely enjoyable as long as you don't constantly think "this isn't how it was in the book!" The characters are interesting enough, the interactions are nice, and the action scenes are actually quite fun to watch. Just a bunch of friends on a road trip, fighting mythical monsters to save Percy's mom. It has its own charm.
Overall, people just don't seem to give this movie enough credit. I am almost sure that if this movie took out all the "Percy Jackson" elements altogether (you know, rename all the characters into something different and make some more changes to the storyline and characters), it could have been something that more people looked back fondly on.