I got one about two weeks ago. It is pretty impressive albeit pricey. Sharpness is to be expected with any macro lens, but what sets it apart from the non-IS f/2.8 is the color rendering. Contrast and color appear crisper with the ED coating. The IS is a real treat since I mainly shoot hand-held. I was able to get usable shots at about 1/60th. You will still need a tripod or steady support for ultra-ctirical sharpness though.
Have you done a head to head comparison with the older version? I have never had any issues with the contrast or color rendering on my 100 f/2.8. At least one review that I read suggests that the two lenses are nearly identical in performance except for build factor, weatherproofing, and IS.