Whats to buy? Its going to be free to download and install, like Steam is today. Yeah there will be SteamOS devices built by Valve or most likely other companies, but since its free alot of people will probably try it. My use case will be installing it on my older pc for a living room based gaming PC.
As for people adopting it, when the SteamOS and the hardware are released you know Valve is going to have a big carrot dangling to get people to try it out, something related to a game with a 3 in the title methinks.