I loves me some Vin Diesel, so I'll watch him anything. But I'm the first to admit he doesn't have a lot of range as an actor, so some of the pathos that his character requires is nowhere to be seen. However, he does a good job selling the action scenes which was pretty much most of what we were looking for.
The visuals are also gorgeous. There feels like real depth in this world they've created, and it's rendered in gritty, creepy detail.
But it's got its weaknesses. The script is thin. There's little subtlety past the opening sequence in the contemporary world. I can't stand Elijah Wood (I don't know what it is about him, I don't think he's a good actor and every time he shows up on screen I just want to poke him with a pin to watch him deflate) so the fact that he's got a pivotal secondary role was annoying. The young witch who helps Diesel isn't bad, but as he starts to warm up to her, all I could think was, "Don't do it! She's jailbait!" Apparently, my grumpy fortysomething self is showing her age. :P