I bought one of these on eBay for my Mamiya 645. I haven't really done much with it, but the exposure metering is really easy, the pinhole cap has the aperture and focal length listed, so I have gotten perfect exposures using my handheld meter. The aperture is something along the lines of f/200. They are available for a bunch of cameras and auto cameras with off the film exposure metering should be even easier to use it with. (Olympus OM-2, etc)
Back to the original question, exposure times were long and I used a tripod, but llong in terms of a half second or so, not minutes. I plan on getting out with it again to see if I can do something worth looking at.