This is such a "two years ago" argument. My upgrade to Karmic was much easier than Hardy or Jaunty (I didn't try out Intrepid). Heck, it was even better than my upgrade to snow leopard.
The perfect upgrade is impossible (i.e across all user's machines); my experience has been only Debian and Gentoo come anywhere near close to painless, but to be fair, that's probably mostly because I'm constantly doing micro-upgrades on these dist's (Deb "testing" track) and there are still some app's or packages that haven't been tested into oblivion by the time I get to them, and so require me to troubleshoot and fix on occasion.
If you care about stability more than being near upstream's head, you have to choose an LTS dist, and you have to vett every single upgrade the Vendor/Maintainer wants to apply. If not, accept that there's a percentage risk which you have little control over.