Took a couple of tries to actually update my Surface (probably because I was running on a spotty-at-best hotel WiFi connection), but it is now running Windows 10. I have done very little on it, but I do like how the start menu is simpler and I do not need to use the Metro UI. I was actually pleasantly surprised with how well the Win10 start menu worked with a touch screen, it's quick and simple.
I've tried using Cortana, but again, spotty hotel WiFi, so I'll give it a fair shake at home before passing any final judgements on that. That said, my experience at the hotel was pretty unhelpful. I need to know if there are some keywords or keyphrases that Cortana can use to voice-launch certain programs. I'd also like it if there was a quick keyboard command or app switch to turn her on or off (there might be, haven't really looked).
Hearthstone still runs well though, so I guess I'm still happy. Oddly enough, it seemed that the Surface ran quite a bit cooler when playing Hearthstone as well, (it can get a bit warm after a while with the fans running, nothing critical, but noticeable), so I'm not sure if that's coincidence or Win10-related.
I'll post more as I use it this week. Later this week I'll probably be ordering a new video card and will be giving Christina my GTX 660, at which point in time I'll install Windows 10 on her computer and reformat to make sure the new video card gets integrated well (and to clean up the system).