I was a late adopter to the 29'er mountain bike size and played around on one for most of a year but it just never felt right-- too cumbersome in the tight twisties, the front wheel felt heavy going up rock ledges and the bike just felt "big" in spite of it being the right size for me.
I sold it a week or two ago and replaced it with an identical size, same brand, same model 27.5" bike and it is spot on for me. I can clean areas on this bike that I struggled with on the 29'er, it's much more nimble in tight sections and going over obstacles I can unweight the front much easier.
26"… too small
29"… too big
650b.. just right!
(plus it's more aerodynamic due to the shorter wheels)