The weirdness of sitting in a theater and watching Crispin Glover, as a mutant monster, reciting Old English poetry -- in 3D! -- is surely worth the price of admission. This movie, surprisingly, is pretty good, and is making me seriously consider teaching the poem in conjunction with the movie -- if only because it makes the poem accessible and because the film gives such simple, useful examples of irony and metaphor.
Beowulf seems, in part, to be tailor made for a screenplay because the original story breaks so easily into the classical three act Hollywood structure. Teaser: Grendel attacks. Act One: Beowulf v Grendel. Act Two: Beowulf v Grendel's mother. Act Three: Beowulf v the Dragon, in which Beowulf dies taking the monster down. The end. In a strange, unintentional way, this structure has already been presented to us in film in the first three Alien movies: in Alien Ripley fights the monster. In Aliens Ripley fights the monster's mother. In Alien3 Ripley fights a four legged version of the monster and dies taking it down. Obviously, in some lost codex, is the fourth part of the Beowulf poem, in which our hero is cloned as a Grendel-human hybrid who teams up with space pirates to defeat Dan Hedaya. Certainly Grendel in the movie Beowulf bears a striking resemblance to the alien that appears in the fourth Alien movie.
Gaiman's script for Beowulf is pretty solid. He looks at the original poem and figures out how to tie the three stories together more closely into a cyclical family romance, so that it avoids being merely episodic. It even gives the poem, written in an era before psychology and the interior monologue, a little Freudian oomph that seems more part of the myth than part of the characters, which works out well, since film, especially the action film, is not good with interior states. The film uses the most basic symbols available -- the sword and the cup -- simply and economically. Beowulf strips down to fight Grendel and a sword hilt in the foreground obscures his genitals. Then he enters a cave that has a shape that will not go unnoticed. Confronting Grendel's mother in a sexually charged scene, she strokes the sword tip until it melts into silver goo in her hands. That was the sex scene. The cup Beowulf took with him, used as a metaphor for the original King's wife in an early scene, returns later and is offered to someone else. We know what is being offered instantly. Beowulf is forced to damage himself in the same way he damaged Grendel. Ray Winston is credited as two more characters in the film besides Beowulf -- for metaphorical reasons. All of this is obvious, and you could complain about its obviousness, but it chimes with the un-anxious simplicity of a poem written before Milton and Spenser (though this is probably a simplification in itself).
The script also handles nicely the differences between the poem and the film. Since one of the main themes of the film is the difference between the poem and the life -- Beowulf, in his old age tells his wife to remember him not as a hero or as a king but as a man with flaws -- when the film diverges from the poem it is no mere whim, but a conscious irony. If you do not know the poem you do not need to, but if you do you will get more out of the changes. Absurdly, but in a fun way, this ridiculous over-the-top 3D motion capture animation film presents itself as "what really happened."
Also, Gaiman does not forget to give even minor characters, like the Queen and the fratricidal would-be priest, character arcs.
I have heard complaints about the technology, that they all look like freakish puppets, but the film embraces its role as a cartoon so well I do not think you can complain about this. I think it actually helps in some ways. The 3D experience seems to me to be crucial here. You may have some resistance to a whole film in motion capture animation, but just the gesture of putting on the 3D glasses makes you complicit in the absurdity, and so you care less about a possible realism. You have on 3D glasses. They SHOULD look like action figures. (The story fits the technology as Toy Story does -- especially years ago, everything Pixar was going to make was going to look plastic-y anyway so why not do a story about plastic action figures?). I have heard complains in which the scene where Beowulf fights naked is compared to Austin Powers -- what random object will obscure his genitals next. I think that joke is being made in more than one place more because it is easy than because it is true.
Plus Angelina Jolie shape-shifts into mutant high heels, even when naked. This is a really fun movie.