why do ppl love these things? what do they do that a canon or nikon can't do?
and why do they cost so much more than normal dslr's?
There's several points here:
Brand: Leicas, other than some of their rebranded compacts, are very precise largely hand built products sold in low volume. They are no-compromise products designed and built with no cost compromises. As a rule, there are much more expensive than the mass produced products from Nikon and Canon, even the top models in the Nikon and Canon lines.
DSLR's: The Leica S2, the only current Leica DSLR, is so expensive not only because of the build quality but also the format. It is a large sensor camera, approximately as much larger than "full frame" as"full frame" is comparted to APS-c. The price of such sensors and the necessary lenses goes up exponentially with size.
RF (M9, ...) vs SLR: Interchangeable lens rangefinder cameras are vastly more difficult to build than an SLR. There is nothing in the focusing system that will adapt for any minor tolerance variation in the alignment of the focusing mechanism nor in the lens mount and coupling on both the body and lens. For over 75 years, every Leica RF lens will focus
exactly the same and will focus accurately on the newest M-series body. The amount of time and effort in calibrating every body and every lens off the production line is significant.
RF vs SLR: Its not something you can really understand without shooting with both for a reasonable amount of time. A written explanation can't really do the trick.
I've shot with a reasonable range of cameras over the roughly half century since I graduated to fully adjustable cameras. I've own a vast number of cameras (over 300 at one time) though only a small percentage were ever used for more than one roll (collector syndrome). I have put a significant amount of film through 7 RF cameras and 5 SLRs, not to mention the various medium format (2) and large format (2) cameras, before drifting into digital. The RFs include four premium interchangeable lens models, 2 Leicas and 2 Nikons. I generally prefer SLRs, but for some shooting the smaller, quieter, lower vibration RFs were preferable. The RF finders were better in low light, though never as accurate.