My two Quadra 950's work great with the all the modern debian kernels.There is one annoying bug that affects my 950. Sometimes the IOP ADB driver gets confused during ADB probing and you end up with no keyboard or mouse. By "sometimes", I mean about 1 boot in 5. I've tried to fix it, but no joy so far. But, if you get past the probe, it seems to be fine. If this is a problem, you could always boot single user, so that the boot process never gets to mount anything r/w unless the keyboard works.
How about re-probing the ADB bus later on (from the booted system) when no devices have been found?
(I recall this wasn't really possible way back when, but with all the PowerPC work gone into the kernel, it might stand a better chance now.)
Anyway, four out of five working isn't half bad. I recall one quirky driver (Mac SE??) that worked one out of five or so.
Michael