Anyways...
The 650b wheel is being touted as a good compromise between the 26 and 29 inch wheels. For some people, the 29er may be too much wheel and the 26er not quite enough. Having ridden fat for several months now, I can honestly say the 26er is far too small of a wheel for me. I've gotten used to being able to roll over things with minimal effort due to the conservation of momentum that the large and heavy wheel provides. Indeed it is harder to get going, but that also means it's harder to stop!
In situations where you need to stop and go repetitively, the 29er wheel may be at a disadvantage to the 26er wheel since the rider will expend more energy powering that wheel back up to speed. Throw in some technical features like logs or rocks and the 29er will plow through while the 26er throws the rider over the bars. From a TECHNICAL standpoint, the 650b wheel size makes sense as a compromise between the 26er and 29er wheel sizes.
Having watched the highlights from the 2012 UCI XC race at Pietermaritzburg several times, I can honestly say that Nino Schurter does a helluva good job making that 650b prototype look good. The guy gets sideways in a rock garden for crying out loud. To the aspiring XC racer, this gives the impression that the bike inspires confidence and that's what a lot of people are looking for. Gotta give credit to Scott and Schurter since they achieved what they set out to do.
Disclaimer: I am in no way trying to state in this post that the 650b wheel is or will be the ultimate be-all-end-all wheel standard (that would be the fat wheel/tire combo ). I am simply trying to compare the wheel standards objectively. As usual, personal preference is key and ride what you like etc etc.