Boy, a lot of mis-perceptions and mis-information out there (and some good information and educated comments as well). Simply put, a 29er rolls faster and clears small obstacle more easily, speed equals wins in MTB racing. There are applications where 26" tires work better, but when racing cross country the 29er is ruling the day. And you ask why do downhill MTB racers still use 26" wheels? Surely they would benefit from a faster rolling wheel/tire. 29er wheels are just not stiff and strong enough for the abuse that a downhill course dishes out.

For commuters? It doesn't really matter a whole lot unless you have a really bumpy commute; ride what you like and like what you ride.

As someone has already stated, a 29er is a 700c wheel, and as someone else has correctly pointed out, unless it's a knobby it's not really a 29er but a 28er. The most technically correct nomenclature is a 622 wheel/tire which is the wheel/bead diameter where the wheel and tire interface.