Gil,
Long time ago, I did a small brainstorming on my website, after having used my M8.2 for a few hours.
It describes the eminent differences at the time, I switched from the R-D1, having loved it very much to the M8.2, which I had a hard time with in the beginning.
This small article is a unique time capsule, as I see many things about the M8.2 now differently, using it as my daily camera for some time now.
I love the M8.2. It is indeed an upgrade image quality wise - you won't believe the difference in clarity and sharpness of the images (not to talk about the sweet spot in resolution for printing).
The files are also smaller, which is nice.
The best thing though about the M8.2 over the R-D1 for me is the infinitely better rangefinder for much preciser focussing of fast lenses in low light.
When it comes to pushing high ISO files, the R-D1 still has a slight edge, getting smaller and smaller with every RAW developer software generation.
The latest Adobe Lightroom versions really have transformed the Leica M into a usable camera above ISO 640.
Here is my small time capsule:
http://www.teknopunk.com/articles/about_the_Leica_M8.2.html
Go for the M8.2, check it for the known issues, to assure yourself, you have a good copy and use it, as long you like it.
Good luck!