There's so much that has been said about Leicas, but at their height and with consistent use, they will become just an extension of your arm and vision. Even with non-metering models, just the entire aspect of walking around, pre-setting shutter speeds and f-stops, and then focus, click, wind.

You can do the same thing with any decent SLR, but there's something so unobtrusive about a Leica that it isn't the same as raising an SLR to the eye and shooting - the latter commonly has that feeling of "picture being taken by camera now."

Consider too the often underrated aspect of Leicas, and rangefinders in general - the effect that the camera has on the subject. An unobtrusive rangefinder can be both unobtrusive to the photographer and to the subject and it at times in has benefits on the photographs themselves.

Aside from that, they're small, can be reliably shot at slow speeds, and are very quiet.