Indeed Chris I have one on loan at the moment. I'm not really a mountain biker but have been out on it a half dozen times and it's brilliant. I have a Kona Wo. Going to do a long cycle along the coast here tomorrow.The main problem is that everyone wants to talk to you about it. A very drunk man outside a pub kept kept slapping me on the back and says jesus fair play, fair play. He then asked me had I consider the Tour de France, I told him I had.
Instead I have opted for a slight compromise version of a fat bike in a 29+ (on an earlier thread here http://ukbouldering.com/board/index.php/topic,23983.0.html) though I don't get it delivered until September time. hopefully it will give me most of the benefits of full on fat bike (compliance of ride with a rigid fork) without the gut wrenching drag from tyres that size.
I think 29+ could be the future alright.