The premise of this movie - that Greek gods were real and have offspring in the modern world - is interesting. But the execution of this idea felt very rushed in this movie. I haven't read the book, so can't judge where the blame lies for this.
The movie is an enjoyable way to kill a couple of hours, but isn't as engrossing as other movies of this genre.
Several big stars feature in cameo roles and for anyone who knows a fair bit about who celebrities are, this tends to distract from the movie. Uma Thurman, Brosnan, etc., aren't really given the screen time to build a strong character for themselves. Perhaps the best of these is Steve Coogan playing Hades, but even here you send most of his screen time thinking "oh look, it's Steve Coogan" rather than being drawn into the movie. It could have benefited from having the minor roles played by less known actors - as it is, their appearances tend to detract from the main characters who actually do a good job with the material they have been given.
But these are minor concerns - the biggest problem with this movie is that Percy finds out he's a demigod and suddenly seems to develop gifts with no effort or training, just by virtue of knowing his past. At the start of the movie, he already demonstrated an exceptional ability to remain underwater for long periods of time. But after finding out he's a demigod, he develops strong sword fighting skills with no training or effort (the start of the fight between the red helmets and the blue helmets, before the water "thing"), and later is capable of manipulating water in a couple of scenes without being taught this skill or having any difficulty or doubts about his ability to do it. This is just preposterous. The mythical/other worldly framework is generally used to provide a context to display human failings, struggles, and virtues. Instead, in Percy Jackson, this framework is used to simply give the lead character a loose justification for overcoming other mythical creatures. He doesn't have to question how to use his powers, nor does he have to put any effort into developing these powers.
Like I say, the movie isn't so bad as to make it unwatchable, but they really missed a chance to make this movie have a strong impact by failing to give it any depth. It is all just the bells and whistles of CGI and having mythological creatures just to have some interesting looking battles and a trip to the underworld. It could have been so much more. Whether this is a failing of the movie, the books, or both, is up to those who have read the books to decide. But the movie reeks of jumping on the Harry Potter bandwagon in an attempt to get box- office success, while ignoring the fact that those movies were built on books that had some depth to them. That is what gave them their cultural poignancy. This movie lacks it completely.
