Thomthumb speaks da truth! Monster truck of the mtb world!
Not as agile, but more comfortable, better for bigger riders, good mile eaters, more relaxing to ride.
They take a bit more 'winding up' to accelerate, but don't tend to get as deflected and slowed down by obstacles as 26ers. That's why you don't see many FS 29ers, and why forks are either rigid or shorter travel. Big travel is inherently less necessary because the wheels and the wheelbase soak more up.
They are great. Probably won't take over from 26ers, but they are a growing niche and here to stay.