I have been using EVF equipped, interchangeable lens cameras, and fitting Leica M-mount lenses onto them, since the Panasonic LUMIX G shipped in 2009. The Ricoh GXR with M-mount module, the Panasonic LUMIX G, the Olympus E-M1, the Sony A7, the Leica SL, and now my present Leica CL are the cameras I've used, successfully and happily, with M-mount lenses in addition to various film and digital M camera.
The biggest issue for me, with *all* of them, is that in bright sunlit conditions, the EVF cannot compete with sunlight and becomes difficult for *my* eyes to see properly. This is at least partially a problem with my eyes specifically ... At my advanced age, while my eyes are still capable of accommodating very low light as well as very bright light, they don't switch back and forth between the two very quickly anymore.
For this reason, a Leica M or other tunnel/optical viewfinder setup remains very desirable for daytime shooting. That is why I keep a Leica M4-2 around, have a passel of other film cameras with similar viewfinders, and am interested in a Pixii or Leica M digital as well. The CL that I use now as my 'main camera' (hard to say "main camera" when I switch around almost daily between five or six different cameras) performs brilliantly in most other circumstances, focuses fast and accurately no matter what lens I fit to the front of it.
But in the particular situation when I'm out in the sun doing "casual" picture taking, my eyes have trouble and the optical tunnel viewfinder, with or without rangefinder, works better for me. That's why I'm interested in (and paid for) a Pixii: Same format, optical tunnel viewfinder, etc. And if it never arrives, I'll look for a Leica M typ 246 or M10-D or M10-M that I can afford...
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