I am slowly saving up for this lens after trying a friend's copy for a month - I was amazed at how rich the colours were compared to my 60mm macro. Plus the IS is invaluable since I like handheld macro.
One alternative however, would be to put the money towards a 70-200mm IS, probably the f4, to gain the upgraded benefit of that lens for telephoto, plus buy a 500D close-up filter to turn it into a macro. I have seen some stunning photos taken with that setup, since that lens is very very good too. Does anybody have experience with that setup to comment on how it would compare overall with the 100L for macro? I would guess it is less flexible somehow, but I don't know in what way.