Having read and enjoyed the entire "Percy Jackson" series of books, our family had been eagerly anticipating this movie for many months. What a disappointment!
I cannot recall the last time I was so discouraged by the film adaptation of a book. Some of the best characters in the first book never make it to the screen (e.g., Ares), The great plot twists in the novel are butchered in the screenplay, if not tossed aside completely. Aside from Uma Thurman, Sean Bean, Joe Pantoliano, and Brandon Jackson, the casting is questionable at best.
If you haven't read the books, you may like it. Otherwise...skip it and relish the magic of the printed word.