If you want to buy a (true 1:1) macro lens in the 100mm class, there are only a handful of candidates. I know the Canon 100 is a very good lens, other people like the Sigma 105, still others especially like the Tamron 90.
But everyone seems to neglect the Tokina, which is according to this review, a very good lens, in the same price category as the Sigma and the Tamron. The Canon is much more expensive.
(edit) Found another review:
And still another:
Anybody has experience with this lens?