I read a review last week, I think it was on the Telegraph site, saying that this is like a Coen Brothers film turned into a novel, and it really is. If you imagine that then you have grasped the book. It is dark, violent, funny and mystical by turns. Its prose style is very like True Grit, and I think it is also reminiscent in places of Mark Twain. I suppose there is a type of dry ironical humour - in which the violent outcast shines a light on social corruption - which runs through American literature and independent films and TV, from Mark Twain to Deadwood, and this is in that tradition. I seem to be saying a lot about what this novel reminds me of, so let me also say that the McGuffin of this book - the thing which everyone is chasing after - is not properly explained but glows golden light up into the faces of those who behold it. That trope may be familiar to you from such films as Repo Man, Pulp Fiction and The Usual Suspects. It's become a kind of stake marker of the North American picaresque. There is an evil hidden man, the killers and the victims, everything having slightly the feel of a mystical allegory, and at the same time a violent meaningless pot boiler.
I like Westerns a lot, and this is a very good modern Western. The author is clearly well read and accomplished, and the whole thing bounds along in a knowing and intelligent manner. Recommended, but warning for quite a lot of gruesome stuff done to humans, and sad death of innocent cute animals.
ETA - The Sisters Brothers was short listed for the 2011 Booker Prize.