I started this so I felt a responsibility to check into it! Re the point about the goggled Summaron dramatically changing the weight balance and handling feel of the M3: having used this lens extensively on an M3 I had never noticed this. But had I simply overlooked it?
I checked a normally reliable reference and was surprised to find the weight of the goggled Summaron 2.8 listed as 310 g.
I weighed mine and in fact it's about 240g. This aligns with other reliable sources who had actually weighed the lens themselves.
To put this in context, this is pretty much exactly the same as a Summicron 50mm v5 black version.
The next stage of my day's work (!) was to mount each lens on my M3 (which is the later version with the strap lugs mounted slightly forward of centre ).
With the goggled Summaron mounted, the camera hangs almost exactly vertically . With the 50mm Summicron it tips very slightly forward . This is because the weight of the goggles on the Summaron is concentrated near the camera body, so the overall centre of gravity of the lens is nearer the body.
In this sense, my conclusion is that the goggled Summaron is absolutely fine on an M3, and in fact better than some other lenses generally thought of as fairly standard I terms of size and weight.
If by "handling" one includes the action of attaching and removing the lens from the camera, then for sure the goggled lens is a bit different but I don't find it really a negative.
And there's no denying it takes up more room in the camera bag.
Having said all of that, for anyone considering getting a goggled 35mm lens for their M3, I would definitely advise trying it first to see if you like it.
It is a bit of an acquired taste for some. But I think it's one of those things that when seeing or reading about it , without having tried it in the flesh, your initial reaction is "that's a really weird and goofy solution ". But in practice it's actually good.
If you want to use a 35mm lens on an M3, this will always involve a compromise.
But for many who have tried it, the goggled Summaron is the best compromise. That it is also a superb lens, as DennisM and 38Deardorff say, adds to the attraction.