A guy I worked with at a camera store long time ago said "If you take a Nikon, a Canon and a Leica to the top of Mt Everest, the only one that will still be working is the Leica. Now, whether that's true is to me a moot point. What I think he was saying is that the Leica is just one the most well made machines of any kind, ever. I have never owned one, but have always respected the craftsmanship of the camera bodies (many years of innovation) and of course the legendary glass, which as I understand comes from one unique deposit somewhere in East Germany (Wetzlar?).
And last but not least is the very long list of great photographers who have chosen to work with these cameras, and the remarkable results they have acheived.