One of the really nice things about the M9 is the size, it's not nearly as intrusive (in public) as a pro body but it has the same if not better resolution.
True.. but it does go beyond that as well... its operation how you connect is also a factor as bhop eluded to. A few of the obvious.. that comes to mind
* Tunnel vision of a DSLR viewfinder versus the open view of a rangefinder
* Composition with a view that is not impacted by the focal length of a lens
* Complete clear view from near focus to infinity at all times even when shooting wide open. Let's you clearly notice obstructions or distractions in the background
Yes.. expensive (I had to sell an entire high end Canon system to afford it). I would consider an alternative... if one existed. The Epson R-D1 was very close but not quite. I could care less of bragging rights... most of the time people don't even notice I have it with me since I carry it in a regular run of the mill messenger bag (not even a camera bag). Many hate the attention... hence why many tape gaffer tape over the red dot.