A number of years ago I did an in depth test with the S120 maxro which incidently only goes down to 1:2 reproduction ratio as opposed to some other true macros, which go down to 1:1. I suspect this was done intentionally in order to provide a good compromise on all around performance from infinity down to 1:2, unlike the Pentax 120 macro, which goes down to 1:1 and is biased in performance for closer distance subjects in comparion.
The Pentax FA 120 macro, one of Pentax's best 645 lenses, held its own again the Leica S120 in close up testing in studo setting. At mid range and especially long distances, the Pentax could only match the Leica in the central part of the frame but its sharpness and resolution on quickly fell off in the outer regions in comparison.
As others have noted, this made the Leica S120 a very good performer throughout it focusing range in part to probably restricting its close up min distance to 1:2. It handles very well hand held, but as other noted too, it had a tendency to hunt in af, especially at close range and I often resorted to manual focus during a portrait session in order to keep up. From what I recall, the Leica lens did not have a focusing range limiter on the barrel, which the Pentax does and greatly aides in quicker focusing its long foxus throw and hunting. Both the Leica and Pentax macros might be a bit too sharp for certain portrait subjects and this is where each manufacturer has another focal length lens more in line with strictly portrait work.
Dave (D&A)