First, the short version: it was a fast paced, man-movie of an adventure (especially in 3D!). Not quite as testosterone-ly epicly big screen worthy as, say, 300, but still a movie well worth seeing in a theater (in 3D!). Really fun.
The longer version: It was good, but won't hold up - even for re-watching in the near term, and certainly not in the longer term as a "film".
Obviously, there's the whole 3D(!) -motion capture thing. Definitely a huge step forward here, especially as compared to Zemeckis' previous outing with this technology in The Polar Express. In particular, I thought they really nailed the close shots (especially the eyes), but in attempting to service the "real" left a lot of the motion looking very stiff. Ironically, it was the big motion action sequences and distance shots that looked the most fake - great effort, but still short of the "reality bar" and likely to look Dr. Who bad in a decade or so. Still - there are moments when you really, completely get drawn in (and then, *whack* get snapped out :P...)
Story-wise, the conceit of the movie (i.e. why the filmakers are "re-visiting" the well known story) is this: Imagine if everything in the original were literally true, but embellished by the narrator (Beowulf himself for much of the tale) and edited for "mature content" by the transcribers (likely cleric/priest scribes) of the epic Olde Englishe poeme. It definitely adds a layer of pathos to the story - filling in the missing "back story", but also afflicts the story with that Soap opera interconnectedness that's drowned action/adventure storytelling (especially sci-fi and fantasy) in the wake of the Empire Strikes Back.
Sometimes a dragon is just a big evil dragon , you know?
(apologies in advance for the politics of the previous link - but, whattya gonna do?)
Still - its was fun to watch, and fun to note where (and why) it diverged from its source. And it was co-written by Neil Gaiman, who, even when just off, is miles better than most...