I tried one only once, many years ago, in a friend's shop when he had one in second-hand. I found it almost impossible to point it in the right direction. Most people trying to use one of those things would look a lot more conspicuous than someone pointing a camera straight at you -- and besides, you'd need to be sure there was no-one irate in front of the 90 degree device too.
Long, long ago I had a WINTU right-angle finder, but that was a waste of time too.
Cheers,
R.