One of my pals asked me which I thought was better: the Panasonic GH2 or the new GXR A-12 with M-mount. I own the GH2 and tested the GXR / A-12 at a camera show recently.
Here were my thoughts:
GXR or a Panasonic is a hard question, I think....
If you were only to put Leica lenses on it, I think the GXR is better:
+ Lower crop factor (1.6x vs. 2.0x) and bigger sensor
+ Better manual focusing options
+ Ability to code EXIF data to custom lenses
- The eye-level viewfinder was good but not *great*
If you wanted to use the Panasonic / Leica DG lenses as well as others then the Panasonic is better:
+ Much broader supply of auto-focus, auto-aperture Microfourthirds lenses from Panasonic, Olympus, and Leica
+ Ability to mount many more types of lenses (Nikon F, Pentax K, Leica R, C-Mount, original Olympus Pen, etc.)
+ More body options (GH2, G, GF3)
+ GH2 eye-level finder is very good (but not great).
I was also really impressed by the Fuji X100 viewfinder but I want it with an M mount rather than a fixed lens.
I was not so impressed with the Sony NEX but there are many people who like them.
The new Nikon 1 is a failure, I think. The Pentax mini series has some nice 'toy' features.
As for used camera lenses, in USA: KEH camera and B&H camera are the best.
In Japan, my favorites are Camera Alps in Shinjuku and Fujiya in Nakano.