According to the Canon web site you get two stops at 1-1.
I have read somewhere that the IS works particulally well when used in servo focus.
Will try and find the link.
Extract from the Canon site
"The new Image Stabilizer designed specifically for inclusion in a macro lens. It is able to correct lens shift movement as well as the angular movement that most IS systems can correct. This provides 2-stops of correction during 1.0x magnification, increasing to 3-stops at 0.5x magnification and 4-stops during normal shooting."
Here are two interesting accounts of the Canon IS macro
http://blog.martinbaileyphotography....m-lens-review/
http://photographyhappenings.blogspo...usm-macro.html