Debian Unstable (often called Sid) is a branch of the Debian operating system designed for active development of new versions of packages. It is considered the most recent, but also the most unstable of the three main branches of Debian. Other software comes with Oldversion and runs under Wine. Xfce Desktop Environments
So basically as stable as Windows can get
The unstable part is just compared to other versions of Debian. Every now and then a package dependency breaks, but usually nothing serious and you can still use the system until it is fixed within hours. It is not like the system will constantly crash, it is still linux.
I have used sid for years without any major problems. Usually only on desktop though.
If you think about it windows up is super stable