You might remember the basic plot from high school lit class: Hrothgar and his Danish kingdom are sorely tried by ravaging monster Grendel; a band of out-of-town Geats led by our title character show up to dispatch same. But Grendel has a mom, and she's a little ticked at that.
A simple story, but the movie script has been Gaimanized to make it more complicated, gray-scaled, and interesting. Grendel is more like the cranky geezer trying to get the rowdy neighbor to quiet down his raucous partying. Sure, he's overreacting a bit—biting peoples' heads off and all—but we're more sympathetic. Grendel's mom is kind of a babe. And the movie seems to want to make a point about heroic leadership making Faustian bargains with their supposed foes. Fine.
The movie is shot in that CGI/live-action hybrid process previously used in The Polar Express. It allows for spectacular effects, but also gives the human actors a creepy Disney-animatronic look all too often.
There are also a number of shots that will tell you, if you didn't know already, that there's a 3-D version out there somewhere. As anyone who's seen Dr. Tongue's Evil House of Pancakes will tell you, those effects look pretty lame on a TV.
But still, it's a pretty good yarn.