Haven't read the books, but I saw the movie today. Was not bad. Entertaining definitely. Nice action and CGI of the creatures and scenery (check out the scene as soon as Charon delivers them into the bulk of the Underworld - stunning!).
A few things I didn't like: Greek mythology and modern setting do not go together. It was painful to see Greek mythology get molded to fit the modern American lifestyle and all that crap. And for those who saw the film, WTF is with the entrance to the Underworld being right next to......The HOLLYWOOD FREAKING SIGN!!!
I also found a lot of it to be cheesy. The actor who played Grover was funny, but Percy and Anabeth were so-so. And their sword fighting was way too clomsy. They needed more choreography.
PS: Roland, I think I know what about lead actor bothered you. It's the same that bothered me. He looks like Zac effin Efron!