Thanks for your post and the link, not sure if the photos there will sold me out, but in all cases it will be a fantastic lens no doubt compare to 100 macro.
The question is, how much do you feel that OS helped for this lens in handheld shots?
Well I can tell you the OS system definitely works but I haven't used it hand held sufficiently to say exactly how much of a difference it makes. My gut feeling (FWIW) is it's on a par with the Canon 70-200mm f/2.8 IS II (which I also own) meaning very capable. Be aware that the 180mm is a heavy lens, compared to the Sigma 100mm macro which I've tried, however the ability to stay further back from the subjects when required and get the shots without scaring the insects etc into flight sealed the deal for me.