PCG-
We've been extremely happy (and well paid) in our use of the S2. The 1Ds examples I have tried are without question superior in all mechanical features. For our needs business realities have to intrude however, and dictate what features are needed and when enough is truly enough. I haven't taken enlargements out to the "nth" degree to compare them because I haven't needed to do so. I'm sure there is an extra margin of "quality" there too, but in terms of our clients' needs, so what? They are thrilled with the enlargements we have made up to 30x40 to date, so we haven't yet found a reason to invest in the 1Ds. For the price of a single 1Ds we could buy three S2's and have a little left over for media. Even if we had the discresionary funds to buy the three 1Ds bodies we would really need in our business we still wouldn't do it, rather we would put the money against the principal of our studio mortgage- a much more sound business decision.
If you can afford the 1Ds, by all means buy it and use it with joy. If you have to make business decisions, buy the S2 and a whole bunch of lenses for the same money. Image quality is there, but the question is whether or not you can contend with somewhat inferior mechanics compared to the 1Ds to achieve it.
Hank