I've had a singlespeed Mongoose 29er for a few years now and have done 1000s of miles on it commuting.
There's no doubt it was very fast but it did leave me quite tired after each run, maybe I just don't have the power to drive the larger wheel or something. Also, it's dimensionally massive, I really couldn't imagine riding off road seriously on it or getting air on it.
I do think 650 will be the wheel for the future of MTB, just bigger than a 26 to have the advantages of the rolling etc but not too big to cause issues with geometry etc.
I'm interested to hear other folks views on this as well as it does look like the 26 wheel MTB is going to be phased out soon with manufacturers stopping to make them etc.
1993 Kona Hei Hei, 2013 Mutiny Robbo X-Sekt, 2006 Rocky Mountain Blizzard, 2007 Mongoose Sabrosa 29er, 2016 Transition Scout, 1998 Mk 1 Standard Trailboss , 2013 Salsa Vaya, Player Special - 2015 Ted James custom roadie