If you hadn't read the books, then it would probably be an awesome movie. My reasons are:
A) It's a great story even if it doesn't stick to the book, but remember if you haven't read the book you don't know that the books are better.

B) The CGI is AMAZING! The monsters are amazing and look completely realistic.

C) The soundtrack. Composed by Christophe Beck "it’s epic and orchestral, all high production values and good old fashioned Hollywood grandeur, eerie and atmospheric and occasionally creepy with plenty of high octance string arrangements clearly meant to inject a note of classy suspense to fight and chase sequences involving CGI beasties" ( found at MovieMoron.com) and

D) It is action packed for the guys, has a little bit of romance for the girls, some history for the nerds ( I can call people nerds because I am one, I mean who takes ancient Greek in school by choice :P) so it has a little bit for everyone.

The characters weren't all that great though. I mean, think about all the deities they casted but didn't even use such as Ray Winstone (Ares), Dimitri Lekkos ( Apollo), and Luke Camilleri (Luke Camilleri). They changed a lot of the character but some of the changes were for the better, such as Grover. He wasn't the Grover from the book but Brandon pulled the new Grover off so well that I didn't care. Annabeth is a little different. In the book she is tough but not as much as she is in the movie. Also in the movie she and Percy's relationship happens way to fast. In the first book they are just starting to get to know each other, but that leads us to the next point. Are they making another movie?

If they were to make a second movie I think it would be hard because they didn't set things up well in the first to make a second. There is no Clarisse so who gets the quest? (For those of you who don't know, Clarisse is a daughter of Ares in the book series and in The Sea of Monsters she takes a quest to retrieve the golden fleece.) Grover didn't get his searchers license so how did he get kidnapped? Being a searcher has been Grover's life's dream and he is determined to be the first searcher to return alive, but that looks doubtful when Percy finds out that he has been kidnapped by a Cyclops and only has a short time to live. I also think that it's possible. The sales show that lots of people were interested in the series and I'm sure it wasn't only the book readers that went to see the movie.

Also, speaking of books, the book sales went up after the movie came out so the non-book readers are becoming interested in the series, and finally if Chris Columbus doesn't make another movie the fans are going to set up a picket line outside of his house. (Just kidding, sheesh can't "know-it -alls" have a sense of humor? Reference to The Titan's Curse) All in all, the movie was good and I think a second one could be made. And made better.