I use M3's, and shoot with both eyes open much of the time. I am right eye dominant, but I tend to shoot with either eye behind the finder, depending upon the circumstances. I tend to keep both eyes open, and when focusing carefully with my right eye, I can pretty easily ignore the info coming from my left eye. If I'm using my left (non-dominant) eye to focus, I have to close the right eye.
The M3's magnification is close enough to life size that it's pretty easy to keep both eyes open and get the "floating frameline" effect while viewing the world directly. Direct viewing, rather then looking at the world through the camera is what RF photography is all about, for me. Of course, with RF cameras built the way they are, this method only works for those of us who shoot with the right eye.
BTW- I have played with the Bessa R3a with it's 1:1 finder, and with a 50mm lens, the floating frameline effect is nearly perfect- it's easy to focus and compose. The M3 works really well, too, and is of course a better built and somewhat quieter camera, and since it's what I have, I use it.