I've had a number of the 100mm USM Macro lenses, and was disappointed in the images I took with all of them. They were fine lenses, but just too difficult to use handheld and slow AF did not help.
When the 100L came out, I was very skeptical and waited for over a year to see what early adapters thought. There were so many positive reviews to its operating ease and effectiveness of the IS that I finally went for a new one. It was even better than I expected. The lens is not exceptionally sharp, its very good. Instead, its strong point is the faster autofocus and the IS. I can walk around, and get close focus shots that are sharp. I almost never use a tripod with it, I may brace my arms for a extreme close shot, and for that type of use, it pays.
I was very pessimistic about the IS, and would have never expected to get handheld images like these. Obviously, I'm now totally sold on the lens.
Baby Robins just holding the camera over my head and taking a blind shot into the nest. Getting this with a tripod would be totally impractical. I did, however, use a camera mounted flash.
Or this bug that was on my front screen door one morning, just grabbed the camera and snapped him. I used my flash, and braced my elbows on the door.