If they can release a Steam Machine with specs that are comparable to the Xbox One and PS4, in a small enough form factor to sit on a small-medium sized entertainment center shelf with room for proper ventilation, at a competitive price, I might just get one.
I do have some reservations about the Steam Machine though due to it's open nature. If it's designed for gamepad use but mouse/keyboard is allowed, mouse/keyboard players will have a distinct advantage in competitive shooters. There's also the similar problem of people with higher spec Steam Machine's being able to turn on higher FOV, giving them an advantage in competitive shooters. If Steam can find some way to add in matchmaking options that allow you to be grouped with other gamepad players and players with similar FOV, they should be able to solve those problems. I also want them to add some value to achievements, like Sony and Microsoft have. And of course there's the problem of Steam OS being Linux based and Valve not yet announcing which games are being ported to SteamOS.