I managed to scrounge up an old Dell Optiplex SX270 at work and decided to mess around with it. I installed Windows XP SP3 originally, but that was boring, so I grabbed a copy of Windows 7 Ultimate to try it out.
Frankly, I was shocked with the results. Overall, it ran much better on the old hardware than Windows XP ever did, and I really liked the interface overall. The only trouble I had on the install was the video driver. Luckily, techbirdy.com had the video driver I needed. I do get some strange artifacts on some of my video files, but everything else worked flawlessly.
Since it is an older box, none of the Windows 7 flashy UI stuff is turned on, but as a day-to-day workhorse it's turned out really well.