I never really got the allure of rangefinder cameras. The huge advantage they had as film cameras were their size/weight advantage over SLR's, and that their shutters were very quiet and some could synch at any speed. Beyond that, why would anyone actually prefer looking through a window that outlines the approximate area of the lens coverage with lines that do not fill it, and have to rely upon a focusing system that is optically more challenging and less direct and slower overall to focus accurately?
In low light, they can often actually be easier to focus than an SLR and especially over autofocus... (and I guarantee, after a dozen years shooting rangefinders, I am faster to find focus than any manual focus SLR, and many non-pro grade autofocus machines too - especially with the advantage of easy zone focusing).
Some people prefer having the area around the frame lines visible, so you can see what will be moving into frame before it gets there - to better anticipate good moments. And of course, the viewfinder never blacks out when you take the shot - you know if you got it.
Another big plus (objectively, probably the biggest) - not requiring a mirror behind the lens means that wide angle lenses do not need retrofocus elements added to the optics, so you end up with sharper, more accurate wide angles than you can easily make for an SLR (and that they are smaller and lighter for much wider apertures is a bonus).
Edited by liamstrain - 2/23/12 at 8:20pm