Very important, if you're hand-holding and not using flash. Camera-shake is magnified in macro and is a major problem at 1:1. Image stabilisation is not so effective at macro distances as normal range, but you'll still get one or two stops of shake reduction, and that's where the most benefit is to be had. Eg, for ease of maths, let's say you have 8mm of blur on the subject without image stabilisation, with one stop that's reduced to 4mm, and two stops takes it down to 2mm, so 6mm total reduction in movement. The next two stops that you might get at longer shooting distances, would only take the movement down to 1mm and 0.5mm, 1.5mm total.
Canon's hybrid image stabilisation in the 10/2.8 L macro is better than most at macro range. Image stabilisation also makes framing and focusing easier.
