I may not have read the books, yet, so I'm not going to spill on how dissimilar the movie is to the novel. Rick Riordan and Chris Columbus' biggest achievement is making the film interesting to teens who may or may not be fans of fantasy, without going too much in pop culture territory nor as spiritual as a Shakespeare play.
There are books out there where the boy or girl hero wants to rescue their parent, usually their mother, after being kidnapped. Aaaawww. However, there's much more in the conflict than that. Whilst doing it, Percy must go after the the X's in their magic map to find the trail that will lead to the kidnapper's lair, while battling foes that are after him because of the same reason Percy's mom got kidnapped; everyone believes Percy stole the lightning bolt of Zeus. You can never eat your cake and have it too, but apparently, the Gods of Olympus can, and when one gets stolen the right, the robber must be punished.
And as hard as it is to admit it, Percy Jackson is very similar to Harry Potter, with the group of two boys and a Hermoine, with the main character going to a special camp. Some may call it counterfeit; I like to call it a "version". After all, there are several ways to write a book in life. Soon, and maybe it's already happened, making up something original will be like trying to make up a new sound; a new letter in the alphabet. Simply impossible unless partially copied. And in this case, the formula still works.
The Harry Potter movies got more and more mature with each one. This part 1 of the book based series is less pretentious than most of the Harry Potter movies, and throughout the story, there are some nice surprises, and I enjoy movies that are unpredictable. And I think I'll give the books a shot, cause at the very least, Logan Lerman and the rest of the cast appears to be having a little fun here.