I sympathize with Keith's agony, but I think I understand. He really enjoys the full-frame, rangefinder experience, and he has a good collection of M-lenses. I can't imagine he would sell the lenses, so the only choices are to live with a crop-factor camera (which he has tried), pay the price of an M9 (which is kind of high, okay really high) or wait for the M10 (which will probably have superior ISO and focussing aids at a MUCH higher price).
I've gone through the same mind-game.
I lucked into some Leica lenses with an M6 a couple years back, and more recently lucked into a deep-discount M9 with some lenses. Rationally, today, with the way the prices have gone up, I couldn't justify either package. I should cash out and replace my PoS car or pay off my credit card, but since I got them cheaply I can justify using them as cameras or investment.
In terms of image quality, I came to the realization that these lenses truly are special. I believe they would draw as nicely on crop-frame, but I'd lose DOF opportunities, and it would be somewhat of a waste as Leica glass draws that quality across a full frame sensor.
In terms of price, I could more easily justify the costs if I sold the M9 and bought a NEX-7. In exchange, that would give me rather better ISO, and perhaps improved focussing aids. I don't care about more resolution. I do believe mirror-less is about to eclipse SLR for all except sports or weddings, which I don't really do.
In terms of ROI, I probably would be better off selling the M9, buying a NEX-7 and waiting to upgrade to the M10, but I have a sneaking suspicion that the M10 will be priced rather higher than the M9, and that M9 prices won't drop that much, that fast. Leica is like that; even the used M7 and M8 are pretty pricy!
In short: M9 is the only Full Frame, rangefinder solution that uses Leica lenses. Aside from the cost, the only cost is a sensor with ISO quality that doesn't match 2012 sensors.