i almost bought one of those in the calumet store in new york, but the salesguy talked me out of it confiding that i could do just as well using a viewing card as mobtown describes. closing one eye helps to flatten out your stereo perspective; you can learn at what distance to hold the card relative to your available lenses by comparing viewfinder/groundglass image to the held card. if you're shooting b&w, the important thing is to then assess the values in your prospective image...and that's the harder part. picasso said: "if you run out of red, use blue." what he meant was that, for him, the value was more important than the color in some situations. in b&w, that's always true.
now i never leave home without my viewing card and usually wonder just what people must think i'm doing walking around looking at stuff that way. so far, no one's ever summoned help!