Username or EmailPassword
All anecdotal - so here's my anecdotal evidence I never used to upgrade but have a separate /home and install from fresh. My previous experiences upgrading have been bad. However, with 12.04 I couldn't be bothered and have upgraded one desktop and three laptops, with out a problem, my sons even has a Nvidia optimus and bumblebee worked after the upgrade I was surprised.
As for Unity I've not had it crash or degrade with updates. I like it it works for me. If you want a distro with Gnome three based on Debian testing I would suggest Debian testing or Mint either the Debian version or the Ubuntu based version.
Canonical is set upon building their own unique OS based on Unity - that might not be for everyone, so use another distro or Ubuntu respins. If you have the time and skill make your own, you can even base it on Ubuntu they provide all the source code.