There is a new article posted on Cyclingnews.com by James Huang of AngryAsian fame. This particular article is an intro to a longer bit of research to try and determine -- somewhat scientifically, he does have an engineering background after all -- the benefits and detriments to 26-inch and 29-inch mountain bike wheels. He makes some good pionts in the article, but that won't keep me from putting in my two cents.
First, let me state this up front, I have ridden exactly one 29" wheeled bike -- I recounted to make sure. There is a reason for this, and I'll get to it in a minute. Second, I am not opposed to them, per se, rather I feel that they -- like single speeds, full rigid, full suspension, road bikes, mtn bikes, 'cross bikes, etc -- have a place. That place might not be under me, that's all.
29" bikes essentially use road wheels (700c) and wrap them with HUGE tires. The effective outer diameter is around 29". As James points out, this alters the geometry of the bikes in order to fit the larger wheels/tires.
This is where I, personally, have issue with them. Often, propenants of the larger tires say that they are better for everyone. Not true. They might be better for me, but I'm at the small end of those that can fit bikes with the large wheels. Now, I'm not a giant at 5'8" -- on a good day, when I'm wearing shoes and thinking tall thoughts -- but I'm not miniscule either. However, adjust a bike for 29" wheels and I get poor standover, longer wheelbases, and generally not ideal handling. It's like really small road bikes with 700c wheels. They don't look right and the angles seem to be off, just to fit the wheels.
While a problem for me, this, I think, is where the wheels shine for taller people. They allow the bike to be sized up correctly. Taller frames, taller wheels. Just remember that everything needs to be adjusted too. Brake rotors need to be bigger, or the same brake doesn't slow you as well as with smaller tires. The gearing will be harder, too. Once everything is adjust, for the larger wheels, though, the bike could be a faster rolling one.
Oh, and the lucky dog, he gets to ride two custom Seven hardtails.