Here's my initial input, although I have only briefly used a Leica that a friend owns. Bjorn will no doubt give you a far more balanced assessment!
In terms of image quality, Leica generally has superior optics to main brand DSLR manufacturers. Due to the lack of a mirror, the lens sits closer to the sensor itself which helps IQ - I'm sure Bjorn will go into far greater detail here...
Most DSLR lenses (there are a few exceptions) offer optimum performance once stopped down. Most of the Leica range perform very strongly wide open. The nearest I have in terms of performance is my Carl Zeiss 2/35 Distagon which is sharper at f2 than my 24-70mm f2.8 pro zoom at f4. Sure, there may be relative 'dogs' in the range, but as a whole the lenses are good performers wide open. That's before you get onto colour rendition, bokeh etc etc
AF is not the be all and end all! Far from it! With my manual focus Carl Zeiss, in certain circumstances, I can get a sharp in focus keeper far quicker than with an autofocus lens.
Hard stop at infinity - check
Distance scales for zone focusing - check
Form factor - the M9 is bigger in the flesh than you may think, but it is far more compact as a system and more discrete.
Shutter speeds - without a mirror clacking about, with smaller lenses and less weight, you can get away with slower shutter speeds hand held
Viewfinder - depending on lens, you can see subjects before they enter the actual shot. In some ways and circumstances, very useful
In many ways the M9 and M9-P may seem outdated when comparing specs, but rangefinder shooting is a completely different experience. I for one will be very interested to hear Bjorn's summary on the M9-P versus the new M and his reasons for disliking the latest Leica offering. There are many features on the M9 range that seem antiquated which for the money I feel should have been updated (LCD resolution being one, for example).
So if you get caught up in the marketing advances (who realistically, as an amateur, NEEDS 36MP, if they only occasionally print at 6x4"?) then the M may seem behind the times and antiquated. However, as a shooting experience I fell in love with it!
Patek Phillipe versus Timex. They both do the job, vary considerably in cost, many buyers of the slightly (
) more expensive Pateks just buy to collect but many more to wear and appreciate for many years to come...