A quick question.
As I have had a stroke in 2011, against better judgement, I have using Testing and Sid packages.
How stable is my Testing/Sid installation?

Testing is stable until something breaks it. How fast it is fixed depends the bug but could be months according to the documentation. Unstable can break often but tends to be fixed faster. If you watch the bug reports and the debian IRC you should get some idea what is happening.

If you want something that is pretty much unbreakable then stable is a better choice but you can't downgrade.

I've read threads where members almost never have problems with testing. It also depends if your hardware is supported by an older kernel etc.

I used Testing for quite a while on quite old hardware and never broke anything. In theory you can get trapped in a situation where there is a serious bug which they have fixed in Unstable, but other things keep popping up there which prevent the fix from being copied over. All I can say is it never happened to me.

Mixing Testing and Unstable isn't the best idea. You really should run one or the other. I prefer Sid (Unstable) because if something does break, it gets fixed quickly. Unstable does not mean that it breaks often, just that there are more frequent updates. Debian Stable is called that because the packages don't change. Once a Stable version is released, only security updates are posted. Unstable, OTOH, gets new package versions regularly. I have run both Testing and Unstable, and I have found that Unstable has fewer serious issues, and the ones that pop up are fixed more quickly, than Testing. Mixing the two can lead to more serious issues, though, because of version mismatch with packages and their dependencies. Seriously, commit to either Testing or Unstable, and don't mix them.