My M3 is hands-down my favorite 35mm camera. I like it so much that I bought another so I'd never have to be without one, just in case one gets lost, stolen, or has to go in for service. I've owned and shot with several other M cameras, but never liked them nearly as much. My M6 flared so much it was nearly useless half the time; the M4-P's finder was slightly better, but for me the camera was still a poor comparison for the M3. Give me that big, bright, high-magnification finder any day. (Both of my M3's have great, clean, crystal clear finders, but I do understand that some aren't as great these days- find a clean one.)
Now- I use a 50mm lens most of the time. If I liked the 35mm FL more, I'd get an M2; as it is, I make do with a goggled 35mm lens. Admittedly, the M3 is less than perfect with 35mm- and this is a really popular and useful FL for many folks. But IMO, for literally every other focal length, the M3 has a better finder than any other M camera. The smooth operation and build quality of the classic M's is just icing on the cake.
If you ask me, Leitz got it nearly perfect with the first M camera, and have just been muddling it up ever since.
(Now, if they could just make a 1:1 magnification finder that could still fit in great 35mm framelines and show good, parallax corrected 35mm, 50mm, or 90mm lines by themselves, that would be my idea of the perfect RF viewfinder.)
