Leica R lenses work beautifully on Micro-FourThirds bodies, like all adapted manual lenses. I particularly like to use them with those mFT bodies that have the high resolution EVF (Panasonic G1/G2/GH1 with built in EVF and Olympus Pen E-P2/E-PL1 with the optional EVF).
Adapters are easily available. Panasonic sells one themself, but you can just go to Ebay, search for "Leica R to G1" and see a range of products available from $20 to $100. Most anything in the $30 and up range is probably indistinguishable in quality. Cheap ones are, well, cheap.
Regards what you "lose/retain", it's like using any adapted manual lens:
- manual focus and manual aperture control only, the body doesn't know there's even a lens fitted. This means you have metered manual and aperture priority exposure modes available, none of the others work well because they presume control of the lens aperture. All metering patterns are available. Focus, set aperture, set exposure, frame and shoot. I often forget the lens was designed for something else.
- none of the camera functions dependent upon the AF system work ... like face detection or face recognition, focus tracking, etc.
- focusing with the EVF or LCD is very good because of the manual focus assist magnification feature. You have to enable this manually, however: it can't switch on automatically when you touch the focusing ring like it does with Micro-FourThirds or FourThirds SLR lenses in MF mode. (Again, the camera doesn't know that you have a lens attached..)
- Remember that FourThirds format is significantly smaller than 35mm film format (13x17.3 mm vs 24x36 mm). This means that field of view is substantially reduced when you compare what is imaged with a specific focal length to what you see through the viewfinder of your Leica R SLR. The difference is referred to as my the misnomer "crop factor" ... a 50mm lens fitted to a FourThirds format camera provides the field of view of a 100mm lens fitted to a 35mm Film format camera, so the "crop-factor" is considered to be 2x.
I like to think of it more simply: where a 50mm lens is considered to be a normal lens for 35mm Film format, the equivalent field of view on FourThirds is provided by a 25mm lens. So for any field of view you want, you need a lens with half the focal length.
This means that your Leica R lens collection, if it went from ultrawide (20mm) to tele (200mm) is going to go from a wide normal to a very long tele on Micro-FourThirds cameras. Ultra-wide to wide field of view requires very short focal lengths not generally found for 35mm Film cameras (the two choices in Micro-FourThirds mount lenses at present are the Panasonic 7-14 and Olympus 9-18).
That's about the size of it. IMO, Micro-FourThirds bodies with their electronic imaging system are the best bodies made yet for adapting lenses to. Of my Micro-FourThirds specific lens kit with five lenses in it, only one of them is actually a Micro-FourThirds mount lens ...
Cosmicar 12.5mm f/1.4 TV (C-mount - covers central square of the format)
Panasonic 20mm f/1.7
Konica 40mm f/1.8
Olympus Pen F 70mm f/2
Pentax 135mm f/3.5