For those not inclined to read the original:
http://en.wikipedia.org/wiki/Beowulf
It's been 30+ years since my original experience with the epic, so I hit the Wiki right before heading over to the Imax.
The movie tells a story using the established characters, and the initial plot and setting. From there, it's a total sellout to Hollywood and technology.
First off, the animation was kinda creepy. The scenery was awesome and the rodents and the wolves looked great. The humans, however, came off worse that a tribe of David Caruso clones: stone-faced characters that can't display emotion, with cold, glassy, staring eyes. I've seen video games with better renditions of human characters.
Grendell is a disappointment, both as a character and as a supposed vision of terror. He's protrayed more as a petulant child than evil force of nature. His appearance was as much dictated by the screenwriter's bastardization of the story line as by any truly nightmarish monster. (if this were a review on the psychology of the movie, I'd compare the appearances of Grendell and the dragon and what that says of the character of their respective parentage)
I forgive the storytellers from ignoring the funeral of Scyld Scefing, the action, after all, really begins in the great hall of Hrothgar. Once Boewulf enters Grendell's mother's lair, though, all relationship to the original manuscript is lost. From there on, the movie becomes just another adventure cartoon, spiced up with the forged nakedness of Angelina Jolie. They try to make some sort of morality play out of it, to the detriment of the moral message of the original tale.
On the plus side, the 3-D rendering was tremendous, although I could have done without the obligatory, unneccessary, "in-your-face" bits thrown in to highlight the 3-D-ness of the movie. And the movie, as a whole, was rather entertaining, if you don't expect the actual story of Beowulf. The action scenes, especially the fight with the dragon, are really well crafted, and could not have been achieved as well with live actors.
At Imax, in 3-D, I don't mind having spent the money. Don't bother on the smaller, 2-d screen.