I was expecting tough times when I started on the Windows 8.1 upgrade trail. Turns out to have been quite easy though (albeit a bit time consuming). The Surface RT upgraded itself from Windows Store, and I was pleased to discover that it had remembered all its previous settings and applications. The Surface Pro was a little bit trickier. For that I burned a DVD and did the upgrade from an external drive. But having said that, the upgrade offered to keep all my programs and data, and was true to its word.
The only thing that us Brits need to know about the upgrade process is that it won't work if you don't have the US English language installed on the device. If you have a UK version of the devices you'll need to change that over before the upgrade will play ball.
The machines seem no slower, although I've yet to give them a proper workout. The Start Button is back, but what really interests me is changes to the display zooming over different displays. With my Surface Pro I need to zoom the display because the pixels are too teeny tiny. But when the Surface Pro drives a monitor it zooms that display too, which is just what I don't want to happen. In Windows 8.1 they have apparently changed the way that displays are zoomed, so that the tablet display and the desktop display can be different, which is much better.