In the beginning, there were 26" wheel mountain bikes. And they were good. After many uncontested years, the 29" wheel mountain bike wheel made an appearance. And they were good, pretty much. The bigger wheels were a fair amount heavier, and the industry didn't quite know how to adapt the bicycle frame geometry to make the bikes handle well, but man did they roll fast and easy over ugly terrain.
The 29" wheel survived the "this is a fad" stage. Manufacturers embraced the wheel size as serious stuff and began to tweak mountain bikes around the new hoop diameter. Handling got better, weight was reduced and selection of components improved. 29" mountain bikes became a force to be reckoned with.
Now, enter the 27.5" wheel, also called a 650B. Use the terms interchangeably. 27.5's have gone through all the same growing pains as the 29ers. They've proven themselves as serious stuff. Most importantly, they absolutely rock in all conditions. They're not just for the super serious riders anymore. This season, we're starting to see 27.5" wheels on entry level trail bikes like the one pictured below. The Trail X650 sells for about $570.00.
Without spending any time on the physics, the 27.5" wheel takes nearly all of the benefits from both 26" and 29" wheels, and leaves behind nearly all of the disadvantages of both wheel sizes.
For a confidence inspiring, smooth rolling, easy handling, back trail exploring ride, make sure to check out the 27.5" mountain bikes this season.