This machine launched before the Steam OS was released which means it is built on the Windows OS. This means all of the games available for Windows are able to be played on the machine. With Windows having a much larger amount of games available over Linux, which is what Steam OS is based on.
Being that this is a Windows OS machine there are some parts of the UI such as the log in screen which are not optimal for controller support. It can be difficult to type in ones password when logging in.
The machine comes pre-installed with Steam OS as the main OS, this is based off of Debian Linux and as such Linux has less games than on Windows. This means that many AAA games will not be available on the system as they tend to be Windows only.
The box is quite small, a lot smaller than an XBone or PS4. It is square shaped and looks like many other set top boxes meant for a living room. So will fit in quite nicely with any other peripherals being used for a TV.