I( switched from the Nikon to the Leica S2, specifically from a D300/D700 and occasionally the D3X, to the Leica. There is a
world of difference, which I will try to briefly summarize:
(1) The Leica is medium format and as such, the depth of field "feels" (and perhaps mathematically is) considerably less than on 35 mm
(2) You lose the 51 focus points, so you either learn to use the one focus point and "recompose", or in reality, merely manually focus
after setting the composition. The large difference is the fact that manual focusing is EASY. The viewfinder on the Leica is SO bright,
and clear, and the focus ring so precise, that it is almost effortless to manually focus (quickly, I might add). That said, medium format
is not a fast action camera.
(3) I find that the dynamic range appears to be significantly greater on the Leica. I haven't done a HDR image since getting the Leica!
(I do mostly fine art landscape, abstracts, NOT studio work). This is a large change in workflow.
(4) The Leica lenses are remarkable, and if your like extreme detail, clarity, micro-contrast, and the "draw" or character of the leica
lenses, they leave the Nikon's "in the dust." (I have the Nikon 14-24, 70-200, 24-70, so the better Nikon lenses).
(5) There are no long telephotos currently available for the Leica, except by way of adapters.
(6) The post processing workflow is simplified because you do not have to "work" as much to achieve the same "micro-contrast",
3-dimensionality. I don't know quite how to express this, but there is certainly a distinct difference in the way the images appear.
You may like the look, or not, but it is definitely not the same as with the Nikon.
(7) The native size of the image from the Leica is 20x30 inches. I moved to medium format because I want to PRINT VERY LARGE
images. Up-sizing from the Nikon is not the same as starting with 20x30 inch images. Furthermore, the medium format images
have such detail, contrast, etc. that they can easily be printed 40x60.
Ergonomics: The Leica feels great in my hand (which is a smaller one), and there was essentially no transition between the
Nikon and the Leica. Indeed, the simplicity of the Leica interface makes the transition essentially seemless. That said, there is
a difference with regard to the LCD. Leica does not have a "thumb wheel" and moving around the image is a "two step" process.
Yes, you get use to it, but it will never be as easy as the thumb wheel approach.
(9) The LCD is sharp enough on the Leica to judge focus.
(10) The Leica "blinkies" show BOTH highlights and shadows (red and blue) and this is VERY NICE. On the other hand,
once the preview is taken down, you can't get the histogram back (hopefully Leica will add this in a firmware update).
(11) The Leica does not allow multiple exposures or what Nikon calls image overlay. I miss this. Again, that could be
addressed in firmware.
(12) The menu or user interface is simplistic -- in the POSITIVE connotation or meaning of the word -- with the Leica. Yes,
there are not as many options, but the essentially ones are immediately available with four simple straightforward buttons
that are programmable as well.
(13) No live view. I rarely used this with the Nikon, so am not really missing this feature.
(14) The Leica battery last even longer than the Nikon D3X and D700 which is remarkable. Even looking and studying every
photo, it lasted two complete days of shooting ! (On the other hand, the Leica battery is around $225 -- it should last !)
(15) The Leica feels the same weight, and feels slightly smaller and more nimble, than the D3X or D700. Of course, I do not
have the optional grip on mine, so the comparison to the D3X may not be fair.
(16) Despite what I had read, and much to my surprise, the Leica handles low light superbly. I recently shot in an abandoned
mill, no light except what came through windows. Exposures were 8 sec at ISO 320. No noise. Excellent image quality.
But, I manually focused. There is no focus assist light on the Leica.
(17) I am still debating whether the Leica exposure metering is as accurate as the Nikon. I think it is easily "fooled", but the
histogram and the bi-color "blinkies" make correction easy.
The biggest difference is what we call here in the US "sticker shock". Lenses cost 2-3X the price of the Nikon. The body
is roughly 3X the price of the D3x. Accessories cost more.
Is it worth it? Every time I begin to question the cost (which is daily), I look at the images and stop complaining!
Hope this helps.