Personally, I don't see that much of a future for them. Short term at least.
Yes they're cheap to buy, but I question much else about them.
The current android game base is not suited to the big screen + controller. They will essentially be starting as a brand new console.
Much has been touted about how 'easy' they are to develop for. They use Java which everyone knows, but the underlying hardware is completely different from what pretty much all developers are used to, so its going to take a while to start getting some nice optimisations.
Have to design around a new, different controller.
Yearly hardware refreshes make development of big games risky, pretty much removing one of the key advantages consoles have over PC (from a developer point of view).
Also, there is some stiff competition in gaming sector ATM. New consoles from the big 3, PC gaming in the rise, steam box on the way, and cloud gaming looming around the edges.
Finally, I already own a powerful android device, so all the other features the consoles tout are redundant.