Magnus is right.
Advantages over SLR and DSLR:
1. Leaf shutter is nearly silent. Very quiet "click" when photographing.
2. Cost - Back in the 70's they were top notch competitors to pro SLR's at budget prices.
3. dedicated lens means lens elements can be matched precisely, giving you razor sharpness and excellent focussing. Very very fast lenses were the norm.
4. Lightweight and simple to operate.
Negatives:
1. Lens attached permanently - only 1 focal length.
2. Old tech, no possibility of autofocus.
There are only a couple digital rangefinders.
http://www.flickr.com/photos/dolespics/3309935419/