Since no one invited me to a press screening of Beowulf (*sniff*sniff*), I'll have to settle for the comic book version. Oh, and the novelization Geekerati is sending me.
WARNING: POTENTIAL SPOILERS
I'm going to be writing about the comic book here, not the movie -- but since the comic book is "Based on the Screenplay," we can assume that most of the changes to the Beowulf story in the movie are found here. If you don't want to know those changes before seeing the movie, stop reading here.
This comic book (really a single binding of a four-part series) follows the poem in a rough sort of way. The basic outline is there: Grendel attacks Heorot Hall, Beowulf shows up to help Hrothgar, Beowulf rips off Grendel's arm, he confronts Grendel's mother, and years later sacrifices himself to defeat the dragon.
It also deviates in some significant ways. The entire action takes place among the Danes -- Beowulf, after arriving, never leaves. One of the monsters isn't killed (allowing for a cliche horror-movie-style ending leaving the possibility for a sequel), Grendel's mother is a sexy shapeshifter, and both Beowulf and Hrothgar are basically jerks. As with so many other adaptations of Beowulf, the monsters are in some way related to both Hrothgar and Beowulf.
I think the changes work, for the most part. A few are ridiculous; for example, at one point Hrothgar needs to be removed from the narrative, and Gaiman and Avary remove him in the most mawkish melodramatic way. The structure of the comic is based around four monster encounters: Beowulf's fight with the sea monsters during the swimming contest with Brecca, the encounters with Grendel, Grendel's MILF, and the dragon. In between we basically have a lot of mead hall singing and wenching.
If this is faithful to the film version, here's what you can expect: Buxom wenches, monsters, more wenching, more monster, wenching the queen, sexy monster, barely-legal wenching, climactic monster, roll credits. Essentially, think 300 with more wenching.
Gaiman and Avary do something interesting in terms of the theme of storytelling that I think works, even though I don't like it. Again and again the narrative returns to the unreliability of storytelling (often represented by song in the comic book, just as in the poem). We keep hearing stories that turn out to be unreliable, and we are left to understand that the poem of Beowulf is one of these unreliable stories: praise for a hero who was less-than-noble behind the scenes.
Since I'm not really a comic book kinda guy, I can't really speak with any authority about the artwork, except to say that I really liked it, and certainly liked it a lot better than the DC Comics Beowulf series from the '70s. My only complaint about the art is that the women (except for Wealthow) all seemed to have the same face, except that this one has braids and that one has red hair. The men are much more varied.
One last note -- Unferth gets rehabilitated a bit here, which seems to be a trend in Beowulf adaptations ... you either make Unferth a terrible bad guy, or you allow him to redeem himself in ways that other characters do not.