Quick Nikon S2 review.... :
1. Leica Ms are much nicer to use as focusing is easier not having to deal with the focus razor wheel (you can focus by turning the lens directly, but need to do that slowly as it is geared to the wheel mechanism and is not designed to handle rapid turns).
2. The aperture rings are goofy as they turn when the lens is focused (not the aperture itself, but the indicator of where it is set if that makes sense).
3. The VF on the S2 is super nice and I think 100%. Not parallax corrected though. RF blob patch is nowhere near as good as on a Leica, but better than on any S3 or SP that I have used. For some reason on those cameras it is less bright and defined. The VF has only 50mm frame lines, but it is perfect with that. S3 is a mess with 3 etched frame lines always there, SP needs two viewfinders and a dial to pick frame lines (and that was meant to be an M rival??)
4. Shutter speed dial is really old fashioned lift and turn, with kinda hard to see set mark. Rotates when shutter is cocked. Leica Ms much better, as are Nikon S3 and SP.
5. Film advance is wikked smooth, Awesome. Perhaps better than an M.
6. Shutter sound is a little louder, but no-one 2 ft away would notice. Shutter button is not in a good spot in any of these Nikon RF cameras. Awkward to use - there is a reason Nikon fixed this when they went from the F to the F2 (I heard the F was based on the RF cameras). M much better.
7. Build quality is awesome. Not as nice as an M, but what is? Way better than the Zeiss Ikon ZM or any Bessa. My S2 is 50-60 years old? And still looks and feels fantastic.
8. Great fun to use. Doesn't seem to need anything like a CLA!
From my experience, there really are three levels of 35mm interchangeable lens RF cameras to look at depending on budget. The Canon P in the $200 range. Super camera. The Nikon S2 in the $400+ range. Then the Leica Ms.