I use a Bronica RF645 as a carry-anywhere camera. The body and 3 lenses are about the same weight as a 35mm SLR kit. A rangefinder does have limitations, but at least it's light and compact enough that I'm likely to have it with me.
The Mamiya 645 is excellent, too. But the flash sync is slow unless you buy a leaf shutter lens. Excellent lenses, but not as quiet and compact as a rangefinder.