Stu it really depends on WHAT you are trying to propogate and WHY you can't provide an adequate set-up otherwise.
I personally think that humidity and possibly bottom heat in cooler conditions is the key.
I have a little thermostat-controlled heat unit, that I can only use indoors. I tried it in a bathroom area, and put it in a styrene box and recycled some old glass over the top and an equally old fishtank light on top of that. Basically to try to kick-start some seeds.
I DID also put in there, a couple of cuttings I had pre-struck elsewhere but didn't want to lose, and they did quite well and have now been hardened off.
But personally I think it IS a waste of time and money- you can create good conditions for cuttings with recycled materials in a sheltered spot with natural light.