I upgraded my Debian 5.0 install to testing/squeeze a few days ago.
The upgrade went flawlessly. I have no idea how the Debian guys manage this but its remarkable!
Squeeze is solid. The software is all up2date. Haven't encountered any real bugs or problems in my day2day use of the system.
What can I say, I love Debian! :-)
http://debian.org/News/2010/20101116b