How bad is it? Dull and pretentious, but different.
Should you see it? Probably not. There are a few people who really love this film, but just a few.
Vincent Gallo plays a priest who goes to Russia in search of a man who went into the catacombs beneath a Moscow church, never to return. The film is mostly shot underground, so it's dark, and there's a lot of interior monologue, so it's quiet, and the film slowly gives hints to the mystery at the center of the story, so it's dull - and there's a lot of artsy shots that make it pretentious. Val Kilmer has a cameo as the gatekeeper to hell. Sage Stallone has a small role. The pretty blonde is Oksana Akinshina of "The Bourne Supremacy."
SPOILER - I think it might be better to go into this knowing what's only hinted at. The children found underground believe that the entrance from above, our world, is hell and that we are demons or ghosts. If you miss that, and most people seem to, the film has nothing going for it.