29 Inch MTBs
These bikes are quite the fad these days. They are a fine choice for a rider, providing they are around 6 feet tall or taller. Here's why.
On the image below; note that to maintain the same 'real' wheelbase and 'real' trail (not ground wheelbase or trail) on an average 23" top tube MTB, the top tube must grow over 1.14 inches, pedal clearance drops about 1.4 inches, steering quickens by over 2 degrees, suspension travel is reduced considerably, and fork and tire selection are reduced to only a handful of options.
Most people are not in a position to give any of these features up for the supposed (and probable) gain of having a larger wheel to roll over the surface easier. By giving up suspension travel, though, this gain is reduced still.
Tall riders over 6 feet can give can free up a lot of geometric space to make these wheels work for them. Notice when a tall person is on a 29" wheel bike, it 'looks' right. When a short person is on one, it look god awful.
Comparison of two actual frames
Here is a comparison of two frames, one a 26" and one a 29" wheel frame. The represent standard issue type product rather than extreme niche product. note that the 29" frame will handle like a truck compared to the 26" version. Also, the manufacturer of the 29" bike recomends this size for a much smaller rider than I have given it credit. I went by comparable top tube sizes (approx 23") that most average sized men will end up using.