I don't think there's anything wrong with the Sigma macro. but I'd also look at the Canon 100/2.8 macro if you're concerned about price or the Canon 100/2.8L IS USM macro if you don't mind spending a few more dollars. The new Canon L macro is especially nice since it has multi axis IS with gyros and accelerometers which optimize performance in the macro range by compensating for both translational and rotational camera movement. As far as I know the Sigma just has the normal rotation compensation IS (OS), which is ineffective at close macro distances, though would be useful for handheld portrait work of course.
Canon 100/2.8 USM Macro - $599
Canon 100/2.8L IS USM - $929 after $100 mail-in rebate (price drops when you add it to the shopping cart as it's lower than Canon's Minimum Advertised Price)
Sigma 105/2.8 OS - $669
All three lenses are very sharp especially since you are very likely to be stopped down when shooting macro to get DOF.