I followed the link, but don't quite understand the product. Does Ubuntu Touch OnePlusOne replace the OS on the device?
So I buy one of the supported phones listed, and then erase and replace Android with this in it's place? Or it installs on top of?
It replaces the OS, so you will lose everything you currently have on the phone. The installation process is similar to flashing a ROM and you can choose to reflash and move back to Android in the future.