Maybe that's a little harsh. The movie isn't entirely brainless. It's just that it doesn't expect very much from itself. It's like it knows that it doesn't have to be great to be a good video game movie. The bar's not set that high.
I never played the video game so I couldn't tell you if it followed the plot at all. I'm guessing yes. Parts of the movie, especially the flashbacks, feel very much like cut-scenes. The music reminded me a great deal of The Mummy and the chase scene through the marketplace is very live-action Aladdin. The plot is super-predictable (I'm going to go ahead and say **SPOILER WARNING** for people to whom it is not) the second you see Ben Kingsley in guyliner. Come on! Brother to the king with a title like Chief Advisor? He might as well be wearing a sign that says "I have dreams of global domination". **END SPOILER**
Prince Dastan is played by Jake Gyllenhaal. I guess he's okay. I just don't buy him as an action hero. And not just because of Brokeback Mountain. I think it's the big puppy-dog eyes. They just don't say raised-in-a-slum to me. Maybe it's the fact that he seems genuinely upset to have to kill people. Has no problem killing snakes, though. I want my heroes to be slightly more stalwart. Ah, who am I kidding? I like 'em just this shy of psychopathic killing machines, but that's just me.
The movie is beautifully shot, the costumes and sets are lavish, and Alfred Molina adds like 10 IQ points automatically, but the action scenes left me just a tad bit annoyed. There was more of the in-too-close, Greengrass-style shakycam that's supposed to stand in for realism but there was also way too much use of sudden slow motion. I think that, like 3D, slo-mo is great in small doses. You can really amp up a cool move by showing it happening veeeerrrrry slooooowly. But the fight scene outside the little mountain village looked like it was edited by a 4-year-old with a pause button.
To recap, Prince of Persia is a fun, summery, popcorn-flick that won't make you any smarter but won't actively insult your intelligence either. And it's PG-13 so there's very little blood and no cursing.