Okay, in that it won't physically harm your setup...but the heat transfer sucks compared to water. Also, some alcohols are very bad for you, and can evaporate through silicone tubing to get into your lungs. Their molecules are very tiny compared to water's.
Oh, and it can disolve some epoxy/glues that hold many system's together.
alcohol will prevent stuff from growing in your system but, like diggrr said, it's thermal conductivity sucks. you could just add some watterwetter to your water and it will stop corrosion and prevent crap from growing.
I don't recommend methanol. It can be very bad news for pets
and you, if you breathe fumes for long.
Nothing you will ever find will be better than pure water for
thermal transfer. If you want to use an alcohol to keep the
monsters out of your rig I would recommend ethanol i.e.
regular drinking alcohol. I suppose rubbing alcohol would
work, too, but I have never heard of anyone using isopropanol
in water cooling rigs?
my college chem prof made me show the 10th grade chem class today some cool reactions that involve fire, I filled a bottle with about an inch of methanol then rub it around the bottle and lit a match over the bottle turned off the light and FIRE +++++ looked soo cool drool...hehe