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. :)
