The script is awful, the plot is silly and the dialogue is just painful to listen to most of the time.
BUT I liked it a lot and am seeing it for the third time tonight and have ordered the DVD from America.
Like all videogame movies it's main flaw was a dumbing-down of the plot, I don't know why the keep doing that.
The film is easy to enjoy if you just let yourself not be bothered by the dialogue or silly moments and enjoy the atmosphere - the sets are some of the best i've seen in any film and for the most part the special effects are top notch.
The creatures in the film are all actors, dancers and contortionists in latex and make-up too with some CG touch-ups, and they look much better than any just CG monsters you'll see.
So yeah, get over the script and let yourself enjoy the film and you should be fine, unless you get a cinema full of assholes talking and laughing the whole time like my last session, but that's life.
I think it's a real shame it's been getting such bad reviews. It deserves a good 3/5 for the sets alone.