If you want a reasonable release cycle, without the drool-proofing of ubuntu, try Debian sid. It's the unstable branch, and gets rolling updates and the like. It is also reasonably stable.
Debian Sid treated me well enough until today, when I decided to upgrade all packages before all the post-Lenny stuff started flowing in. libxcb-render-util0 had previously been upgraded to Experimental's version when I installed Awesome 3, which was available only in Experimental. After removing Awesome 3, I was unable to revert back to Sid's version of libxcb-render-util0 (as well as libxcb1, which had also been upgraded) without removing a great number of other packages. After forty minutes of messing around with aptitude and dpkg without making headway, I said to hell with it and replaced Debian with a minimal install of Ubuntu.
Sid has also done some odd things in the past couple weeks, such as removing slocate from the repositories. It took me a few minutes to figure out that I ought to switch to mlocate. The primary drawback before that was the delayed security updates – due to the Lenny release process, the Firefox (well, Iceweasel) 3.0.5 upgrade took perhaps two weeks to enter Sid, leaving me with the vulnerable 3.0.4 for the period. Ubuntu seems to offer swifter updates.
A minimal install of Ubuntu is very nearly as lightweight as Debian. It provides a stable base and a clearer upgrade path than Sid, albeit at the cost of packages that are behind the cutting edge. Security updates for any packages in Ubuntu's main repository seem more timely than for those in Debian Sid; however, a great many lesser-used packages in Ubuntu's universe repository get no security support at all. Overall, given the issues I suffered when mixing bleeding-edge packages from Debian's Experimental with a Sid-based system, I believe Ubuntu is a better fit for my needs.
Regardless of which I settle on, both are a great deal more fun than Gentoo, which I ran for five years before switching to Sid in September. The novelty of being able to upgrade all packages in five minutes instead of fifteen hours still fills me with joy.
It's usually reasonably stable, but occasionally there will be severe breakage when major components are being updated and reach the repo's at different times. This happens more with X, Gnome, KDE ... but sometimes when major system libraries are upgraded bad things can happen.
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. :)
