The big wheels on a 29er take a little more motor effort to accelerate, simply because you're turning a lot more mass there... but the reward is that you hold your momentum a whole lot better, and because the wheel is bigger, you can roll over stuff much more easily and a lot smoother ride. I find I get a lot more traction as well, which makes it easier to climb and to get through tight/slick places.

I have both a Specialized hardtail and a Gary Fisher full-suspension 29er in the stable, and strongly support what Bike4Ever said about the Genesis geometry and the way it improves bike handling for both their 26 and 29-inch models.

Size-wise, the rumor is that 29ers are for big/tall folks, but the reality is that if the bike fits you, and they do make 29ers for smaller folk, you'll like the way the 29ers ride.