The hybrid IS system works best at long ranges, delivering all four stops of stabilization Canon claims, though it ironically—given we're talking about a macro lens here—falls short at close-ups. But more than that, "this is one of the very finest lenses we've seen - optically it's superb, and operationally it works very well too, with fast and positive autofocus, and one of the most effective image stabilization systems currently available." In other words, it kills the current non-L F2.8 100mm Macro.
It'll be interesting to see this new system migrate over to other, non-macro lenses. Check the full review there: [DP Review]