We've tested a wide variety of alternative lenses on the S2 over the past several months. We have the Pentax 400mm EDIF and it is a good lens on the S2, although the 350SA is noticeably better. The 350 CF non-SA that Al owns and Mark G borrowed for his Everglades trip is very sharp, but does show a fair amount of CA, even when stopped down. The Pentax 400 isn't as sharp as the non-SA Zeiss 350 wide open but has zero CA. It is also a full stop faster. The Zeiss has more inherent contrast vs. the Pentax, but this can be boosted with a simple S curve in LR.
We haven't tried the 500 Zeiss or the 600 Pentax yet. Most of the interest seems to be in the more manageable 350-400mm range.
when Leica releases their 350mm f/3.5 APO S lens in a year or two it will most likely best any of these lenses. If we assume similar performance to the 180 and 120: With fast max aperture, high contrast and resolving power, APO correction, amazing sharpness with smooth bokeh, weather sealing, auto aperture control and AF, the Leica lens will probably be one of the best tele lenses ever made for MF. Of course, they have to make it first.
For now, the alternative lens route makes a lot of sense. These lenses hold their value extremely well and have taken their depreciation hit years ago. Buy it, use it until the Leica glass comes out, and if you decide to go for the S lens, sell the alternative lens for the same price you paid for it. And, in the meantime, you have a focal length that isn't currently offered. Seems like a good proposition. This might be why so many of my customers have done this, and not just for telephotos. With a wide 82mm diameter bayonet, short flange to focal plane distance and a focal plane shutter, almost any MF lens can be mounted on the S2 (except the ones with fully electronic apertures).
David