Last year, when I finally decided that it was time for a new bike, I test rode the Santa Cruz Tallboy and the Yeti SB95. Both were great fun - I rode the SB95 down the single track trails on Burke Mt. in VT with a huge grin on my face the whole time - but I ended up with a 26" Yeti ASR 5.
In the + column for the 29" bikes:
They go over everything.
Awesome climbing & fearless going downhill.
In the - column:
I love twisty, tight, technical trails and I found the 29" difficult to maneuver.
They make things too easy, all you have to do is point & pedal. Now don't laugh at me when I say that! but I really enjoy the challenge of finding the right line & the victorious feeling when I finally clear a tough section.

If I lived in an area with long, flowy trails, or if I lived on Burke Mt, I'd have bought a 29", but I am loving my regular, old 26" bike. :-)