I owned the legacy Canon 100mm f/2.8 macro lens. While there was absolutely nothing wrong with that lens, I found myself missing hand-held shots at times because of the lack of IS. I sold my old macro lens as soon as Canon released this L-series derivative with IS.
The new lens does not disappoint in any way. It's razor sharp throughout the entire field from f/2.8 upwards, with IQ similar to or perhaps even surpassing the older lens.
But the IS is what separates this lens from all other macro lenses without it. I can easily get two stops of IS, and sometimes as many as four stops. That can make a difference between "missing a shot" and getting one. And at the end of the day, that's what really matters. I'm very happy with my purchase.