I did the Windows XP to Windows 8.1 upgrade on my four-year-old Dell workstation. It works pretty well, and supports a range of really ancient applications either natively or through compatibility mode. I've only found one thing that would not run at all, and that dated from the late 1980s!
But there's a gotcha.. I upgrade to 8.1 via Windows 8. The first step from Windows XP to 8 ran pretty smoothly, all of my data from the XP installation was moved to a folder called windows.old where it could be recovered from by someone with a basic understanding of PCs. All well and good, but the obvious next step was to upgrade to Windows 8.1.. a bit trickier as you can't do that without installing KB2871389 first (either through Windows Update or manually). The Windows 8.1 download is enormous, 3GB+ but it installs smoothly enough.
The catch? Well, upgrading from Windows XP to Windows 8 creates the windows.old folder with the old data in. Upgrading from Windows 8 to Windows 8.1 DELETES that folder and creates a new one with the old Windows 8 settings.. obliterating your original data from the Windows XP installation.
Well, that wasn't a problem for me as I'd backed up everything onto another drive which I unplugged to be on the safe side. But it wasn't what I was expecting to happen *at all*.. and you can see that a less paranoid customer (or one without a suitable backup disk) could well lose everything if going from XP to 8 to 8.1. And I do notice that there doesn't seem to be a Windows 8.1 Upgrade version available anywhere, so this is the path that a lot of people would take..