I've read a lot of poor reviews three star reviews and I would say that's a slightly over-achievening assessment. It's a shame really, though while it's a good enough movie it had the potential to be better.
The framing device of the movie is unusual, it smothers the pace of the opening ten minutes, but when you see the conclusion to it, you want these scenes to have been a bit longer as they turn out to be a great part of the movie. The problems lie in the pacing. Although I did want the climactic scenes to be longer, the film is already over two hours as it is, and instead of those scenes being short, they could have lost some of the runtime from somewhere in the middle portion of the movie, which on occassions drag beyond belief.
Somehow the tale manages to be both very simply, and overly confusing. It feels like sometimes you need a pen and paper to keep track of names, species, locations and where exactly anyone is going, but at the same time the story is pretty simple. Acting wise, no one here is ever going to make the A-list, script wise, it's clunky, but there is something about the daftness of it all, and the final 15 minutes where it all comes together makes it worthwhile.