Upgrading from Windows 8 Pro wasn't a pain. It took some time to download it, but that can also be due to the server and the time (around dinnertime). Installing was pretty easy and I did not have driver-problems at all. I didn't even have to reinstall my x86 programs, only my apps, downloaded from the store.
Upgrading from Windows RT 8.1 preview to Windows RT 8.1 was a bit more difficult. However, with some patience and several retries, I managed to get it working.
After the install, everything works like a charm. I am really happy with it (both versions).
There are a lot of small improvements, which enhance the total experience. The store is neat and clean, the integration with SkyDrive is superior to Windows 7 or 8 and the replacement of the messages app for the Skype app works neat. I like the added features to the settings charm and I also like the way to get to the address bar in IE11 MUI style: just move the pointer to the bottom of the screen and a small grey line will appear. When clicking on it, the address bar and all open screens will come up. No need to swipe from bottom to top.
The only downside I can see at this moment, is that not all apps are fit for Windows 8.1 (RT). App makers should be working on that. For others, some vendors should update drivers to improve the experience for their users.
All in all, I think Windows 8.1 (RT) is an update/upgrade worth downloading.