I've successfully upgraded to 8.1 from 8 using the MSDN ISO.
- The upgrade process was very smooth, no problems or crashes, kept applications and docs (though it told me I had to reinstall VMware Workstation for some reason).
- Changes to the Start screen are an improvement, and the return of the button makes it much easier to use VMs with a mouse.
- Really shitty move trying to force you to use a Microsoft account to log in to the machine. Luckily there are two workarounds: (1) Install offline, or (2) A (hidden) workaround: give it bogus info (email@example.com) when it asks. However...
- Skydrive is all or nothing: either you log in using an MS account or link one to your local user, or you cannot use Skydrive except with the web app. The desktop application is gone and won't install or execute. Yes I know why they say they've done it that way... integrated blah blah seamless blah blah...
- System backup (the Windows 7 style one) is gone. All you can use now is a combination of command line stuff (at least that means you can automate it) and File History. Better than nothing but a step down in my opinion.
All in all... nice upgrade I suppose, but the cloud stuff shouldn't be pushed in your face, and some useful things have gone for no good reason.