I gave the Windows 7 beta a try. I've deliberately skipped Vista, as my PCs are a few years old now, and an upgrade is not a high priority at this stage.
Thankfully it happily installs into a partition of its own, leaving XP intact. Though interestingly once running it didn't seem to even see the original C: drive, but could only see the D: drive it was installed on, but as C:.
To co-exist with XP, it creates a new boot menu, where XP is the “Previous version of Windows”. Not sure how/if I'll be able to get rid of that later when I zap W7.
The installation appeared to go smoothly, but it hasn't recognised key parts of my old PC: the on-board ethernet, the video, and the sound. So apart from there being no net access, no sound, and the video resolution is screwy, it's all fine.
Of course that may all be fixed in the final version.
So I had a play around in it. The new applets (Calculator, Paint etc) are much nicer than the old versions. The whole interface looks pretty nice in fact. And with just the basics installed, it seems to run quite well on my old computer.