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I've used the EF 100 f2.8 non-L macro for near 15 years; film and digital, and just recently picked up the new L IS version. My physical disability sometimes leaves me with slightly shaky hands and the IS helps a ton.
That said, I see little difference in sharpness between the two lenses. The L IS has better bokeh, it's AF seems just a bit faster IMO and balance is better. If you need the IS and are willing to pay 500 extra for it then get the L, otherwise you might think on getting the non-L, used they are about 400; that's a sweet deal.
I have an 85 f1.8 that I use for portraits a lot, the 100mac with IS makes for a nice portrait lens also...easier for hand holding.