I called mine "looper" because on steep, technical climbs it always wanted to loop out from underneath me. I put a different stem on it which helped on the climbs, but then it was uncomfortable the rest of the time. Same thing for my old Rize. It doesn't upset the geometry much on paper, but I sure felt it on the trail.
I just pulled the trigger on one. I needed a front wheel for my '09 Team Scalpel and had to choose between a 26 and a 650b. I decided to give the 650 a try since I ride my Flash 29'er most of the time these days.