I've just gotten the Leica finder to use mainly with my on the way ZM25. I also wanted the option of putting the 28 on my M5- I don't see the 28 lines well there. I initally thought I'd use the entire .58 VF for the 25, but find that even with the 28 lens I lose too much of the VF for my needs, and those 28 framelines are closer to the edge than I was expecting. My only experience with aux. finders is the CV/Rollei 40 and the Contax G 21. I must admit to being a little disappointed that I don't get a Leica-like view, there are no frame lines, only a window opening, so I don't see more than my film area. In use, for me, aux. finders are fine- I've not minded the two eyepieces one to focus one to check framing- and with these wides that check of focus is not going to be as critical in much shooting. I don't do much close-up work with the wide lenses, so I'm not feeling the need for the apparently incredible R4A VF, and I've always been one to make an extra negative or two if I have doubts on framing- even with the 50 I have moments of uncertainty exactly what's going on film- usually not an image breaker, but if in doubt I just cover myself.
I will certainly peer through an R4 when I get the chance- the .58 really opened up my working with the 35 lens a whole lot, and I'm hoping the Leica VF will be enough to do this with the 28 & 25, lengths I was very attached to in my SLR days. If not then maybe the R4A is in my future.