Although I'm more interested in the next gen Xbox console/PC where I'll be able to have an all in one gaming device (Xbox, PC and Steam Libraries + higher end hardware), these new products are wonderful new options for so many people who loving playing games. Competition for Sony and Microsoft and Nintendo is very welcome.
I think it's awesome that Valve is going to release dedicated PC hardware that Developers can target and optimize for since it's a specific set of hardware just like a console. Bonus: it's a compact PC too if there's a need for it. The ARM-based SoC headset is interesting since it's opens up the ARM platform for SteamOS and games which hopefully leads to future more efficient handhelds for example.
Personally I prefer the console convenience over the PC hassle: No worries about hardware compatibility, OS is usually very stable, system and game updates are simple, boot up the console and game with little settings fiddling required, play on a big TV and from the couch and use it with a controller (first UI). However I like to play certain games modded (almost exclusively a PC thing) and PC games tend to be cheaper or on sale more frequently. Both Valve's Steam Machine and the next gen Xbox console/PC will give me all that on one device.
And for Linux as a plattform itself, all of this is also awesome, more ressources poured into it, more development and optimizations, increasing the userbase.