I'm not a big mountain biker. I've had one for years, but I tend to ride 95% of the time on the road, either on my road bike or my tourer. When I ride the mountain bike I usually tend to ride on the local dirt roads. I've done very little single-track riding.
I'm tall - 6'4". I bought a 23" frame Rockhopper with 26" wheels a couple of years ago. It's a hardtail and I put a nice Marzocchi fork on the front. I like it a lot.
This summer I'm riding the Great Divide trail for about three weeks to a month, loaded with camping gear. People told me this was a perfect application for a 29er, especially for someone my size. Well, this whole 29er thing had intrigued me so I decided to go for it. I like getting new bikes. What put me over the top was finding a 23" frame on Performance's website for $99. I pulled the trigger, then wondered, "What have I done?"
Anyway, I finally got it built up enough to ride last weekend and took it on one of my local favorite dirt roads. My initial impression was that it felt huge! However, it rode okay, and the gearing I chose was low enough to make the climbing about as easy as on my Rockhopper, even though this bike is a bit heavier. I think most of the added weight comes from the bigger tires, tubes, and wheels, as well as the discs. (The Rockhopper has V-brakes.)
Where it really seemed nicer than the 26" bike was on the descent. As I got going faster and faster the bike seemed to absorb the bumps, washboard, etc. just a little better. Was the difference worth the cost of building the new bike as opposed to just riding the Rockhopper? No way. But honestly, the biggest reason for the project was that I love working on bikes and building them up. When I don't have a project going I go out to the greenhouse/shop and wonder what I can do. Also, when I hear people touting something like 29ers I like to try it out for myself. It's an expensive hobby, but much cheaper than something like antique cars.
That was a quick impression. I may have more to say after I return from the tour in August.