I'm glad I'm not the only person who thought it was completely underwhelming. My girlfriend liked it, and we don't often disagree on movies.
My gripes
1 - When the best performance is delivered by Angelina Jolie, you may have a problem
2 - Performance capture is great for giving more life to a non-human CGI character like Gollum or King Kong. But the CGI isn't good enough to make people look real, so my brain still won't accept it. To me it's just vaguely creepy and distracting.
3 - Too much "doesn't this sword look cool in 3-D when we shove it in your face"
4 - If this particular movie had been done live-action, it would have been rated R. I can handle a whole lot of violence (I think Sin City is a fantastic work of art) but I was not prepared for people being torn limb from limb and having their heads smashed based on my expectations of a PG-13 movie. I don't understand why this kind of violence is PG-13 because it's just slightly not real, and R if it's real. The content is the same. The MPAA is stupid.
5 - I fell asleep after about 1.5 hours and woke up just in time for the end. I believe this is because the poor performances and vaguely creepy, not quite realistic CGI/perf-capture kept me from forgetting that I was in a movie theater. I was never drawn into the film, and therefore I got bored.
I believe there are movies where 3-D could work beautifully. Superman and Beowulf were not those movies. The Incredibles I would see again in a heartbeat if it were re-rendered with this new process and released to theaters. Having seen the teaser for Coraline before Beowulf, I think that will work wonderfully as well. So far, though, I have seen no finished work that excited me.