Yep, already owning a complete MFD system, and starting from scratch are two completely different scenarios. However, it points up that if a swap where to happen, then you get really serious about whether you would gain or lose anything in IQ. Emotions run high about these MFD choices, but when you are laying out YOUR big wade of cash, things get really personal and sober. This is true for any ultra-expensive system swap, not just swapping out for an S2.
This emphasizes the need to try before you buy. However, there is even a caveat to that trial exploration ... both big players in this field, Hassey and Phase/Leaf, have proprietary software which is widely acknowledged as being part and parcel of the system to achieve the best IQ. IMO, it is VERY difficult to properly utilize these software solutions in a quick cursory examination unless you have prior mastery of them. Once at least partially mastered, these software solutions are as important (if not more important) than the camera system itself.
So, complete, relatively unbiased reviews like the one Jack and Guy did are absolutely invaluable.
As to dealers, again, almost as important as the hardware. The best bet may be to locate previous buyers of any given system in YOUR area and see what their service experiences have been. Not some brand new buyer that will justify their recent purchase decision, but someone that's been a user for some time (it need not be a user of the latest-greatest version either .... service is service, and at some time in future what you buy today will NOT be the latest-greatest). That may be more difficult with a new system like the S2 for two reasons ... 1) there are fewer users and they haven't had the camera very long ... 2) Leica could well concentrate on any issues like crazy during the Honeymoon phase of the new launch .... but long term service support and updates is what you really need to know.
In the end I didn't want to be a beta tester for the camera system, and more importantly I didn't want to be an alpha tester for the service .... which has been less than stellar in the past with Leica's flagship bread & butter product ... the M. Could well be a completely different story with the S2, but I'll let someone else find that out.
-Marc