I was going to get one but decided to try building my own steam machine since steam os can be downloaded now
The issue with prebuilds not from valve is that valve cares about SteamOS to work on their hardware the most. Sure it will probably work but prepare for pain. That's why it makes sense to buy SM if you want SteamOS. It's kind of like steamos on other handhelds than valve. Sure it works but buttons are wrong, shit breaks from time to time and so on.