This may be an obvious post, but I want people to know about this.
29ers are 700c mountain bikes. The tires would be like 700x60s. Big tires. Big rims.
The 29er technology is finally catching up to the frame builders who reached their stride really quickly. If you haven't ridden a 29er in the mountains, you need to.
They roll over things more easily.
The foot print that the larger tire makes is much larger (25% and up)
Climbing is easier because of the last 2 listed
Because of the size of the tires and wheels, suspension is much less necessary for the rough stuff.
Still not much of a selection on tires and forks
In tight singletrack techy sections, you may find you need to go slower to clear quick turns.
All in all, if you are over 5'7" you may be a candidate to go faster with less effort. We are getting in Kona Unit 29ers for demo and I would love to show off the ti black sheep that gets parked at the shop. We believe that if you go on a familiar trail that is not ski lift style, you will go faster. The builder for Black Sheep went about 15% faster on his favorite loop in Fort Collins, on his first ever 29er ride. That was enough for me to be impressed.
Favorite 29 tires: Maxxis Ignitor
Favorite 29 frames: Black Sheep
Favorite 29 rims: Salsa Delgados (I hear rumors of Mavic soon...)
Favorite 29 fork: Black Sheep (lots of healthy flex)
Have fun. Go ride.