I had a waist level finder for my F3 about four years ago, but sold it after about a month.
The screen and the image inside are very small so, the way I see it, its only purpose is to use it for macro/copy work on a tripod, or to be able to frame the shot without raising the camera to your eye. Any sort of focusing that doesn't entirely rely on DOF, even using the flip-up magnifying glass (which is terribly small), is very hard. Unless your eye is pretty much pressed against it (which, in my opinion, would beat the purpose of working with a waist level finder unless it's on a tripod), you can't keep the eye, the magnifying glass and the part of the image you want to set into focus, aligned, because even the steadiest hands and head still move enough to throw it all off.
It's ok for "shooting from the hip" because it allows you to frame the shot better, but, at least in my experience, that's as far as its usefulness goes.
