Casting of the leads was a little strange. In the book, Percy's twelve years old. For the movie, he's aged several years - I don't think his age is ever stated but I'd guess 15-17 range. Book-Annabeth, his female sidekick, is also twelve. She's athletic with "princess" blonde curls. Movie Annabeth is a dark-haired, busty Amazon warrior played by an actress in her mid-twenties. Granted, if you make Percy older than Annabeth will naturally follow...but this is not a woman who still looks like a teenager. She looks, acts and sounds like a grown-up. The choice to use her makes no sense except that in the land of Hollywood, sexy sells.
If you confine the plot to the movie - that is, as a self-contained story not related to any books or potential sequels - it's OK. It's a clunky, predictable coming-of-age story, but it's entertaining. When you compare to the book The Lightning Thief, however, the movie is so disappointing. Major plot changes were made. In the movie, for example, Percy's told his father is Poseidon right away; in the book, a big part of the plot revolves around the fact that Percy doesn't know who his father is, and is waiting for a sign from the gods to reveal it. Percy's biggest rivals at Camp Half-Blood are the children of Aries, especially a girl named Clarissa. She's completely absent from the movie. Aries himself makes a brief cameo at the end of the movie, but that's it. His meddling in Percy's quest has been erased...as has the 'deeper evil' that motivated him and Luke to act as villains. No mention was made of Kronos. Instead there's a weird scene in Hades where Persephone, played by Rosario Dawson, attempts to seduce Percy's friend Grover and saves Percy's ass from the wrath of her husband. It was icky.
I mean, who is the audience for this movie? Kids generally read about characters a few years older than them. To borrow an example from my childhood: when I was in elementary school, I read Sweet Valley Twins, a series about twin seventh-graders. In junior high, I read the high school series for those characters. By the time high school rolled around, I had moved on to Sweet Valley University, although by the end of freshman year I'd outgrown Sweet Valley** entirely. But my point is this: the kids who are reading The Lightning Thief will probably be in late elementary school/junior high. But I think that many parents would balk at having their ten-year-olds see this movie.
Visually impressive, man those special effects have come a long way, blah blah blah. I'm not the type of person to gush over special effects. I mean, at times scenes seemed very blatantly computer-created, but that doesn't really bug me that much anymore. If the story gets told, I'm satisfied.
** Ohmigoo. They're making an adult series with the twins in their twenties. JUST LET THE STUPID TWINS GO ALREADY.