If I decide to get a mountain bike, I'm on the fence now. After taking my CX bike on what's supposed to be an easy MTB trail, I can see where those big wheels are both an advantage and a disadvantage! (I realize a CX bike isn't going to be quite as nimble as a MTB!) I'm tall enough that I can find a 29er with not too much difficulty, so that part isn't a problem. The 650b sounds intriguing, though I'll wait a few years so that there's a good selection of models and tires on the market.