I've managed to get my Kona 29er out for a few decent off-road rides now and I have to say I love every bit of it.
We went out for an assault on the local MTB park last weekend on a perfect, clear mid-winter's day. The singletrack was sticky, even boggy in parts, and the wheels just drove me through it all without slipping. In fact, that's the main thing that has impressed me - the enormous traction. I'm climbing stuff now that would give me trouble on my 26er.
I was also bombing down the rocky parts much faster than I would on the 26er. The larger wheels and rollover just sort of inspired me to hit the harder stuff. I definitely felt more in control.
One thing that did come up at one of our rest stops though was the odd pairing of 6" rotors on larger wheels. They stop me fine, but the torque on the spokes must be more than a 26" wheel. Got me thinking, I wonder if there is a market for 29er hubs with larger flanges so you could use 26" spokes on the rotor side.