FWIW, I shot a goggled Summaron for 2 years on my M3 and really liked it. I did not mind the extra bulk on my camera at all. To be picky, one thing that was a little annoying was the aperture ring is a little hard to grab when you have a hood mounted, but that goes for the ungoggled lens as well. As long as the goggles are clear they will not darken your finder. They will make it a little lower contrast since the lenses in them are not coated. All this being what it is, I can't say how one would be on your M9. I think I had a good experience with mine on my M3 because these goggled lenses were designed for the M3. The M9 already has a lower magnification viewfinder, so it might not be so good with one of Leica's goggled 35's.