My BF had a bike built for him that was a 29er and he just never felt right on it. So he built 650B (27.5) for it and he loves it now.

I personally have tried a 29er but that was at least 2 years ago. I didn't like it at the time, mostly because it felt like it was big and handled awkwardly, especially for cornering. I think that companies are constantly improving the geometry. I still ended up buying a 26er for my last frame. I love that bike. It's tight, corners well and I can fly downhill. I wasn't willing to give up cornering for rolling over stuff, especially since I don't really have trouble with rolling on the 26. I did notice that getting my fork suspension dialed in on the 26 helped a lot with the feeling that I was getting bucked off obstacles with I was climbing.

I am intrigued by the idea of building a 650b bike myself but...