I had a 45mm C (oldest version, 77mm thread) but I only used it on film, years ago.
It was sharp in the centre (even wide open...this is on film, mind) but aberrations got progressively worse towards the edges and corners. Mainly astigmatism, field curvature and lateral colour...some improvement in the former two on stopping down, but not completely cured.
I now have a 45mm S (67mm thread, middle version between C and N: best of both worlds perhaps as it has the refined optics of the N with the all-metal construction of the C). Used on my 37mm square back [with almost exactly the same image diagonal length as the Leica S], this performs very well indeed. Again, it's sharp in the centre wide open, actually really sharp...while the off-axis performance tails off much more gradually than it did with the C lens. Stopped down, I can't really detect any issues, although I haven't rigorously tested for them.
I haven't used a 45mm N but it is optically the same as this S lens. Coatings might be a tad different.
I have a 55mm N and like Brian says, it's awesomely sharp...but not wide open: a pronounced haze of spherical aberration surrounds a sharp PSF core, robbing the image of contrast, and off-axis there's major astigmatism as well. But take it down to f4 (on a cropped sensor DB or a DSLR) or f5.6 (on full-frame 645) and it's an entirely different performer - the aberrations melt away.
Re. the 35mm manual focus...the optics are unchanged from C to N (only the coatings change). I have the N version. It has an imaging personality rather like the 45mm C above, in terms of aberrations. Field curvature is really noticeable. Stopping down for more DOF tends to hide that. If you are picky, I would say that you are much better off with the (also much faster) Leica 35mm, if you can afford it.