I've been running Windows 10 on an old Surface Pro 1 and it works a treat. I installed it from a USB key. I really wanted a clean install but the Windows 10 setup insisted on doing a (very successful) upgrade of the existing Windows 8.1 installation. However, by forcing a refresh I managed to get an empty machine to play with.
It's very snappy. Boot times put my Windows Phone to shame and it seems very complete and stable. I upgraded to the latest version so I could try the new "Spartan" browser. I was interested to see if it could render my blog home page and the editing environment that Squarespace provides. It works great. Takes a while to wind up but once it is going it all seems fine.
I've also done lots of things that I wasn't expecting to work very well, like connecting to network printers at the office and installing strange usb drivers. So far Windows 10 is looking like a win.