I have a 100mm f/2 Canon that I use on my R-D 1 for lecture-podium shots and the like (see attached example.) My success rate is pretty good at distances of about 10 meters and up; at closer distances, it gets riskier the closer you go, although I'll almost always try it if there's a chance of getting a good shot.
As others have said, the R-D 1's short RF base makes it very challenging to get good focusing accuracy with longer focal lengths at wide apertures -- but an eyepiece magnifier helps, and so does using the split-image focusing technique. (To do this, you find a hard vertical line in the subject and focus by lining up the split in the line at the edge of the rangefinder patch; the eye does this more accurately than it aligns two coincident images as in normal rangefinder focusing.)
In the example picture (of a guest lecturer on Impressionism) I used the split-image technique to focus on the frame of his eyeglasses. I was shooting with the 100mm lens at f/2 at a range of about 12 meters, and in the second image (a pixel-for-pixel crop from the first one) you can see that even at this long distance DOF was pretty limited -- the far side of his face is a bit softer than the near side. Still, most of my pictures of the lecture were pretty well-focused. Of course, it's harder if you're dealing with a more mobile subject!
