The $1995 Leica 50mm f/2 Summicron-M is a honey of a lens in every respect. I’ll have more to show on that soon in my Guide and Review of the Leica M9 in DAP. In short, Leica got everthing right with the 50.2.
The 50/2 is the only Leica 50mm lens for the M9 which has near-zero distortion— this makes it suitable for purposes where faithfulness to the original is a goal, buildings, man-made structures, even horizons, etc.
The main usability issue with the Leica M9 is that it’s fiendishly difficult to frame an image precisely, as with the image below, so I hope Leica someday comes out of their coma and add Live View to the M9, or at least a 1MP LCD that can zoom to actual pixels and quickly, so that one can check focus. The M9’s LCD is awful, and the frame lines are highly inaccurate, suitable only for “loose” framing, not for precise compositions. Better yet, a Fuji X100 style design, which has both frame lines and high-res EVF would be super, but that would require a CMOS sensor, as would Live View.