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Just a data point, but I have also been going through this process of finding a Macro for my a900. First I bought the Sigma 70 Macro but, like your Sigma experience, there was just something missing. Technically, there was nothing to complain about but it just did not wow me so I ended up returning it.
Next up, I decided to give the Minolta 100 Macro another try. A few years back, I picked up the original first AF Minolta version and ended up not so happy with it. It would hunt and hunt for focus really bad. Overall, I found it not as good as the Minolta 100 F2 AF I owned at the time for everything where the distances overlapped. It was great thought for really Macro stuff but not so much for anything else due to the focus hunting issue. Anyway, this time around, I picked up the last Konica Minolta D version of the Macro 100. This one may be identical to the current Sony version though it is curious that the the Sony version does not mention anomalous dispersion glass whereas the Konica Minolta version did. Anyway, right away I noticed a huge improvement with regard to the focus hunting issue the earlier one displayed. Much, much improved to the point the lens is useable for anything, particularly with the limit switch engaged. Bokeh is very nice as well - much better than the Sigma 70 above at normal portrait distances for instance. The lens is sharp from wide open but my feeling is it's not quite as sharp as a few other lenses I have used, notably the Minolta 50 Macro and the Minolta 100 F2 AF. There is CA and some fringing wide open, noticeable in torture type lighting situations. Color and contrast is neutral whereas the other two Minoltas just mentioned tend to have much greater contrast. So, overall, it's a fine lens but it still might leave one wishing for more. Unfortunately, that something "more" is likely going to run two to three times the price and likely lack AF. If your flexible money wise, I would probably suggest the Leica 100 APO Macro R as well.