I have been using Ubuntu 10.04 on my old laptop since few months ago, since i wanted to test my Ubuntu repository at my campus. At least that was my initial purpose of installing Ubuntu on my old laptop. If not, i wouldn't replace my Slackware on it
Since Ubuntu 10.10 is going to be released soon, i decided to try to upgrade to 10.10 Beta (it's still in beta at the time i was upgrading it) and it took me about one full day to upgrade (some package were missing and i left my laptop running at home while i was working at the office, so i resumed it after i got back from work).
At the end, it was upgraded safely and no harm. Everything works just like it used to be. Wireless connection still intact (in fact, i didn't get any delays anymore after connecting to my access point just like i had in 10.04), the boot and shutdown process is getting a little bit faster, and everything else just work out of the box (including NVidia driver). I just love it when the process is going smoothly. Right now, i'm updating it daily and it has reached RC release. Since i will not be available when the final version gets released, i am trying to make it up to date as i can so i won't miss too many updates when i return
But still it's far away from Slackware. I can do a full upgrade in less than 10-20 minutes using Slackware. But since i'm running -Current, i don't have to do any full upgrade any more. All i need is to follow -Current and i will be running the latest version of Slackware all the time, but that's another story.