The above reviews focus very much on the director's bold move to have this film entirely animated - this film has been cited to demonstrate the "uncanny valley" theory, which posits that as simulations of humanity reach near-perfection they will suddenly become very hard to sympathise with, because while being very accurate, they are noticeable more for their slight flaws than their all round veracity. In any case, the animation can be a turn-off for some, but it also gives the film an otherworldly sense very appropriate to its subject matter. Grendel speaking Old English is a very nice touch.
The full-bodied and earnest representation of Dark Ages Denmark, warts and all is very engaging, and the liberties taken with the poem all contribute to making this a better film, I think.
The largest deviation from the poem is the insertion of a very Modernist sense of doubt - Beowulf and Hrothgar are both given flaws not present in the poem, implicit in their relations with Grendel's mother. The film is constructed in such a way that it could be seen as the "true" account of Beowulf's life, which a contemporary Bard would have constructed into the heroic poem which survives today - we see that Beowulf's account of his "battle" with Grendel's mother is in fact false, and the Dragon being Beowulf's son is a very interesting twist on the tale, which, in my opinion, enriches the final encounter,which seems somewhat arbitrary in the poem.
A word of warning, however. This film is probably the most sexually explicit and gory 12A around, which the producers probably got away with only because the film is animated - be wary of showing this to under-12s.
In conclusion, this film raises some interesting questions about the original poem, and presents the ancient tale in a more complex, more modern light.