Interesting posts, BM. The Sigma 150 macro was the one I was all set to buy until the 100L IS came out and wrecked my plans!
As always, The Digital Picture review is instructive:
http://the-digital-picture.com/Revie...ns-Review.aspx
Note that the Canon lens that Bryan compares the Sigma with is the old non-IS non-L 100mm macro, not the new one.