Anyone who uses Steam's Big Picture Mode is already intimately acquainted with SteamOS, as they're very similar.
SteamOS looks and acts like Big Picture Mode, except it's the basis for the entire hardware system. It's controller-friendly and easy to navigate.
The same Steam splash page washes across the screen when it launches, and the same tile-based layout of games and the Steam store are visible at launch.
As promised, the OS is built on Linux (not based on Ubuntu, we're told, but entirely custom), though you'd never know it as the only interactive layer is all Steam.