I really liked Grover in the movie. Even though he didn't match the Grover in the book, he still had a lot of funny lines and always kept the mood light. I thought that Logan Lerman was great as Percy too - he looks exactly like Percy and his acting was a little too angsty, but still good.
The only character who really annoyed me was Annabeth, which is sad since she's one of my favorite characters in the books. First of all, the actress didn't look much like how I imagined Annabeth; in fact, she looked a little like Fergie at some places. Plus, she was wayyy meaner than she was in the books - I could never imagine the book Annabeth purposely hurting anyone during Capture-the-Flag. The rest of the characters ranged from good to ok - Chiron was awesome and all the gods were meh.
The plot was different, but still interesting. I mean, it's only a two hour movie and they had to cram a lot of stuff in. The whole overarching plot of Kronos rising to power was cut out, but that's understandable.
The fight scenes were my only other problem. I don't know much about sword fighting, but what I saw of it in the movie looked really choreographed and cheesy and fake. Annabeth kept raising the sword above her head and kept managing to block other swords even without looking back - I thought that was pretty unrealistic.
But in the end, I thought that the movie was worth watching. Different from the book, but still entertaining.