All macro lenses are adequately sharp, so make your comparison on other features. My notes:
http://postit.rutgers.edu …%5FWork%5FDistance4%2Edoc
I've tried the Sigma 150mm macro f/2.8, and next to the Canon 180mm macro f/3.5L, it may be one of the better "affordable" conventional macro lenses on the market (It's no $2,000 Leica, etc.) with longer working distance.
It is better than than Sigma's own 180mm macro (>image contrast, >image color, and >user handling). Sigma's own 105mm does not have internal focusing elements. So, the 105mm, while sharp, is not in the same handling league. In our time, it makes sense to buy internal focus macro lenses - barrels do not change length during focus (all the recent Canon). If you are only using macro occasionally, then something like a Sigma 105mm, Tamron 90, or better the newer Tokina macro) are fine. For frequent use, go internal focus like the Sigma 150mm.
If you do not photograph from a tripod, get a short macro.
My opinions FWIW. Jack