I thought it was a solid movie and the CG was amazing you would think that it was live action. It stayed true to the poem until
Spoiler (Highlight to read):
Beowulf bones Grendel's Mother, which became a major plot point, also I'm not sure but I think in the poem after Beowulf ripped off Grendels' arm he followed him back to the cave and killed him there.
I think they did a good job designing Grendel, seeing as he was never given a detailed description to begin with.