yes paul, i have that exact setup and it's a fantastic combination, the marco elmar produces incredible images, erwin puts recons it' one of leica's best! and the "look" of the out of focus areas is really great. just wish there was a 50mm that could match it.
oh, and it's pocket sized!
I have the (discontinued?) collapsible 50mm also. It's not in the same league, even when used on a Sony with the Kolarivision ultra thin cover glass. It's basically a tessar design, which really can't cut it at 50mm compared to what we expect today. At f8 all but the very corners are up to top notch M standards, but at 2.8 it's a bit soft across the field. At 4 and 5.6 the center is great, but after about a 25mm image circle the quality drops. It may be the best 50mm f/2.8 tessar ever made, but the ZM 50mm f/2 (maybe the best 50 double gauss) simply blows it out of the water, and the size & weight difference is not that significant.
This is getting a bit OT, but let me emphasize also that the Sony cover glass is rather thick. Leica M primes less than 90mm will be affected negatively. My 75mm f/2.8 Leica M shows corner smearing with the OEM cover glass, whereas the 90mm Macro Elmar M is clean at the corners. (The WATE is OK with OEM cover glass, but better with the Kolarivision Ultra Thin glass.)
Also, with the 90mm Elmar I recommend using the Adobe Raw Converter's profile in conversion as there is some color fringing. Compared to most lesser lenses it's not a big deal, but with M primes one expects more, and the raw conversion profile gets you there.
Paul
www.PaulRoark.com