If you are not familiar with the concept behind 27five, (650B for off road) then you might want to check out this post I did on it. Here I want to give some of my own personal views on the subject.
While an additional wheel size to choose from is a point to consider, I also believe that what the wheel size offers in the first place is perhaps even more important to consider. I mean, who cares about a choice if it doesn't do anything better than what we have? Here's my take on what 27five might have on offer. Disregarding any "marketing hype" that has already been spouted, by the way.
The reality is that 27five won't be "better" than a 29"er, at least in the areas that make owning and riding a 29"er "worth it". Think about it. A 29"er has roll over capabilities that are due to it's larger diameter. All the good things about 29"ers are first and foremost tied to it's diameter. Therefore; a smaller diameter wheel will have less of what is good here. My take: If you like/love 29"ers, you most likely won't be impressed with 27five.
Okay, what about where 29"ers supposedly are bad? Tight cornering, acceleration, and weight? Well, if that is the case, (and all of those points are legitimately up for debate) then why not ride a 26 inch wheeled bike? It's quite obvious that a high end 26"er will be able to be had right off a bike shop floor today that will score highly in those categories. My take: A 27five will never be able to out do a 26"er in these areas if a 29"er can not either. Again, the traits are tied to the wheels diameters here, so logically this has to be the case. And the point about 26"ers having been hashed out long ago and having a superiority in choices can not be overlooked either.
Finally, following this to it's conclusion, you have a choice in 27five that can either be seen as a compromise between 26 and 29, or "the best of both worlds", depending on your outlook. Certainly 27five will have some features that are attractive, but my question is will it be a difference that is noticeable? Is the mix of some big wheeled features and small wheeled features going to be a good one, or will it just "muddy up" the outcome? Only ride time will discern this for me, and there is something in the works that if it comes to fruition, will allow me the chance to try out my theroies for real on a trail.
After the determination has been made about performance, a determination about the choice can be made. It's always good to have choices. As long as they make sense, anyway. Only time and public opinion will be able to help clear that up. In the meantime, lets ride!
It's all just bikes anyway, right?