Joey Hess had a BOF about Constantly Usable Testing at Debconf10 last week. I like the idea a lot, and here's why: I run Debian's testing distribution on my laptop. I like it to be reasonably up to date with current upstream versions (if only because the lenny versions often don't work with the hardware), but I don't want to take the risk of breakage I get from running Debian unstable.
In fact, I don't want to the take the risk of breakage I get with running Debian testing, either. CUT seems to me to be an opportunity to get the benefits of running testing without most of its risks.
Now, the breakage I'm concerned with isn't the kind of breakage that makes the machine as a whole unusable. I am more worried about the kinds of small changes that will inevitably happen that I can work around, but that require more time and effort I like to spend.
For example, it might be that when upgrading this morning, something changes in Firefox, which requires me to restart that. Or there's a new kernel that requires a reboot. Or the update breaks Evolution, and I'll need to wait for a fix from unstable for a week, or have to backport it myself, perhaps by installing the relevant half-dozen packages directly from unstable.
None of that is a disaster, but it's all a hassle, and it's all a distraction from what I really want to do.
Having resonably frequent pseudo-releases, cuts, based on testing, would mean I only need to worry about things breaking when I upgrade to the next cut release. Meanwhile, I will have months of stress-free Debian use.
I could almost achieve that by only uprading my laptop every few months to whatever happens to be current in testing at that time. However, there's no security support in that case. The big, central, blinking improvement with cut releases (if I understand them correctly) is that I would have security updates if I stay within the cut release. The security update would possibly just require installing the relevant packages and their updated dependencies, but that's good enough.
And that's why I like cut, at least as an idea.