There have been a number of good reasons mentioned as to why the Leica rangefinder is a good camera. Mostly, I think it is that it is small, very fast, unobtrusive, and makes extremely good images. A camera that one can take anywhere and shove the lens in a persons face during a conversion and get a picture without them even noticing it.
Specifically:
images are better because of the close lens to sensor distance,
The shutter is very quiet,
The viewfinder is bright, always open, and one can see around the photo,
Rangefinder focusing lets one focus exactly where they want,
Much faster than an SLR when taking a picture & no shake from a mirror,
Small and it goes with me every day,
Leica does not use an anti aliasing filter--better sharpness than Nikon/Canon,
Colors are different, although may not be as accurate,
Uses the best lenses in the world--Leica, Zeiss. Many Canon users don't use Canon lenses, but choose Leica R or old Contax/Zeiss SLR lenses for IQ,
and the Leica M just feels good in the hand.
Nonetheless, I have a Sony DSLR w/Zeiss 85mm f1.4, etc. for times when the Leica doesn't work: like very low light, Macro shooting, telephoto work, etc. But, I love shooting the Leica M8 and seeing the incredible images the most.