Installed it last night on my Surface Pro (1, 64GB) as an upgrade from Windows 8.1, rather than a fresh install. Initially, it was a bit janky, as you would probably expect from an upgrade + technical alpha, but after a couple of Windows updates and restarts it seems to have settled down nicely.
As a Surface / touch user, I'm not completely sold on the new windows start bar. I should probably clarify that I never had any issues with the Metro UI in 8.1 on either my Surface or my desktop, as it was easy enough to use for touch and get away from on a desktop. I can see that once I've pinned my regular programs it might be just as good, if not better, but as a few others have mentioned the long list on the left feels like a step back from swiping up to reveal everything installed.
Also the loss of charms for a tablet user, especially to adjust brightness, is frustrating. However, I prefer the look and idea of the notification centre; if they just add a box to adjust monitor brightness I probably wouldn't have any complaints on this front. Having modern apps in a window feels a bit weird ... I think this could be improved by making the title bar disappear when you've got it snapped, and only showing when you move your cursor up to the top of the screen (like in 8.1), as losing out on that screen space on a Surface really hurts.
Quick query for any other Surface users who are also using an external mouse, have you found a reliable way to disable the touchpad? It's in the settings menu, but when I turn it off it stays on ... there also doesn't seem to be an option to disable it when an external mouse is connected. Not complaining, I am more than aware this is a technical alpha and features might not be working / implemented yet, just wondering if it's something I'm overlooking before I submit any feedback.
Always fun to play around with new tech, will be interested to see how it develops.
