Installing was a pain - had two moments were nothing was happening and I thought I have to kick it.
And, No!, it won't erase your entire drive - you can select your partitions. If you ever setup your partitions manually, you can do that do here, too. Installer suggests main, recovery and swap but one was actually enough (have 16GB RAM - what's a swap area, lol)?
Behaved nicely with occassional hickups (downloads, crashes in certain games) - SteamOS itself was always running. Steam controllers work OOB. The desktop is there (gnome) and works. You can set the volume but it never remembered it.
Gameplay seems to be better than in my old Mint (means it was fun to play).
I might actually keep it until SteamOS3 (hopefully soon with Debian Stretch)