For me, testing is the sweet spot. I have run a mixed Lenny/Sid (mostly Lenny, with the occasional package pulled from unstable, currently it's all Lenny) for a long time now and not yet had a serious issue that was not my own fault.
Testing is also a rolling release, but you've got one more level of protection in unstable for serious stuff to get worked out before packages get to you. There is also a useful tool called apt-listbugs which will display information about serious bugs during the upgrade process before packages are upgraded and allow you one last chance to abort the upgrade.
It's pretty commonly thought in the Debian community that if you're running Debian on a desktop system (as opposed to a server) then there isn't much reason not to run testing.
I will continue to track the new testing in due time. I'm kind of going to miss Lenny though. :)
Sometimes it's fun to be on the "bleeding edge", such as when KDE4 first went from experimental to unstable. Plus, it benefits Debian as they've more users testing potentially breaking packages and filing bug-reports.
I used to run unstable for a while and it was by and large still more than stable enough for general use, regardless of its name.
But as I got busier on my own development projects I stated to dislike the occasional issues that would pop up here or there and take my attention away from what I was really interesting in. Hence why I am now using testing with as little pulled from unstable as possible (usually that means nothing pulled from unstable).
I still like to contribute in my own little way, filing/helping with bug reports on the testing release. Just recently I was pleased to be able to assist the fglrx-driver maintainer (and myself) with testing a fix for a reported bug.
If I wasn't so busy, I'd probably still be running unstable, it's still more stable than a lot of distros out there. :)
Testing is also a rolling release, but you've got one more level of protection in unstable for serious stuff to get worked out before packages get to you. There is also a useful tool called apt-listbugs which will display information about serious bugs during the upgrade process before packages are upgraded and allow you one last chance to abort the upgrade.
It's pretty commonly thought in the Debian community that if you're running Debian on a desktop system (as opposed to a server) then there isn't much reason not to run testing.
I will continue to track the new testing in due time. I'm kind of going to miss Lenny though. :)