I upgraded my Debian install last week, from Sarge (stable) to Etch (testing).
The main reason for doing this was to keep up-to-date with various bits of software. In theory I could compile it all manually, but that's more work and doesn't always work anyway, as my failed attempt at upgrading KDE a few months ago proved.
Etch uses x.org rather than xfree86, and I'd been having some issues with that involving X exiting as soon as it started up. That was happening far too often (about 1 in 3 attempts), and was getting annoying as I couldn't figure out what was causing it.
So anyway, I did the upgrade. And X.org decided it didn't want to talk to my mouse. A bit of modprobing and reconfiguring later and it's working nicely, albeit without a functioning scroll wheel. I'll fix that at some point though.