I was super-excited about Win 7. I downloaded the RC and ran it for months, it was nice. It's basically Vista with some minor changes, but admittedly better--when it works as it should.
I convinced my dad to purchase and install Win 7 64-bit OEM on a new build we did together for his personal use a few months ago, using all-new and current components. We had lots of random little issues with it, but the straw that broke the camel's back was that we could never get printing to work. We would try to print, and get an error message that the print spooler service was not running. Verified in the services app that it was not. Checked its dependencies, made sure they were all running, tried to start it: it wouldn't start. We even reinstalled the entire OS: print spooler still never ran automatically, and in fact would fail to start when manually told to do so, all dependencies running fine.
We gave up and bought fricking XP for that computer, and it's rock-solid though he's essentially wasting nearly a gig of RAM we paid for (and he hasn't yet let me configure a RAM drive to utilize it).
As for upgrading existing PCs: if they run Vista, it might be worth it to some people to upgrade. If it runs XP from the factory, it's probably old enough so that upgrading is really a bad idea--the hardware's too old to see any real performance gains, and it'll likely be a headache.
MsDucky: unless there's an app that you need to run that's not supported under XP, I'd strongly recommend against upgrading from XP.