That depends so much on the plumbing... if it's the kind where you have to adjust the temperature every time you turn it on, I wind up wasting as much water fiddling with the temp as I would if I just left it on. Replacing faucets is a little farther than I want to go, until they wear out naturally.

I really like the showers they have at some campgrounds and school gyms - where you press a button for a few seconds' worth of water - but I suspect that there's a lot going on behind the scenes to keep the temp constant in the mixing valve, that would make it wasteful in a home setting.