On servers. A lot of servers. Everything else (well yes, everything including RedHat, SuSE, *BSD, Solaris and of course #@!dows, except maybe Ubuntu LTS) is just a nuisance. You want them to work, and you want them to have all the necessary security-patches, but you don't want to fuss over them every day.
Otherwise, on my workstation and notebook, I use "unstable".