Wow--I just installed Ubuntu on my "new" desktop (a PIII 450mhz) and it's very very nice. Installation was refreshingly easy--I didn't have to tinker with anything. Then I unison'd my user directory over from my laptop and ... tada! I'm productive.
I highly recommend it to myself. If you're like me, then you'll probably like it as well.
But you don't have to take my word for it. Having said that, and as most of these reviews will point out, this is a pre-release. If you check the Ubuntu forums, it's pretty clear they put this out to get feedback for the final release.