Steam Machines will have a hard time for sure, they're kind of an inbetween product that are neither here nor there, in my opinion and I haven't really seen much incentive for me to want one yet, and that is coming from possibly the biggest Valve fan in the Northern hemisphere.
My initial and main concern is the pricing though; they seem to place themselves above the One and PS4 in price on average, you get far more powerful hardware but that is certainly not a guarantee for sales (see original xbox for an example) and the somewhat wonky gamepad will likely turn a few people off, that also symbolizes the inbetween nature of the product for me; it ends up somewhere between traditional and touch and might very well miss both.
Gabe Newell and Valve; I love you and everything you do to bits and I consider this company to be the most positive influence on gaming in general in the past 20 years, along with the likes of CDProjekt, but I think you're making a big mistake with these machines, I simply cannot picture a sizeable market for them.
I almost feel like their goal isn't to sell a LOT of steam machines.
Just to sell enough to convince a number of people to make Linux games.
Why? Windows 8 and it's apstore and a feeling MS might just try and shut down other shops