27.5 is the pajamas. If that had been the 'next hot thing' after 26", 29'ers never would have become so popular. Keep in mind too, that when the average rider got a new 29'er, they were also likely getting way better shocks, brakes, etc etc. It's hard to see the forest for the trees when comparing wheel size when so much else in the comparison is also new/different/improved. And of course, objectivity about an item has an inverse correlation to price paid for item.
I owned a shop for a number of years, and build my own frames. As near as I could, I did a 'scientific' test between 27.5 and 29. Riding loops that I have ridden for years (and timed) over a summer switching between the two sizes. Basically, the lower the angle and the smoother, the more the 29 shined. On what I call 'zero momentum' climbing ie climbs where you stop moving forward if you stop pedaling, the 29 really lagged. Zero momentum and technical was the worse performance. Like some testers mention re full suspension bikes, I often felt very slow on the 29, when in fact I was just as fast, and on some loops with road stretches at both ends, faster. I never liked the feel though.. just felt like more work... like being taken for a ride instead of doing the riding.