I have the lens and love it! I have read reviews that it is a super slow focuser. Keep in mind this is only when using it for macro work. I have found that otherwise, it is quite fast.
Are you looking at getting the old Arc-Form Drive version or the current Ultrasonic Motor version? I have the AFD version which is a bit old but quite good.
Phocus, please also consider the EF-s60mm f/2.8 Macro for use
on you 20D. I have the 100mm too but on my xxD bodies I've always
found the 60mm more useful as it's faster focusing and has a more
pleasing (to my eye) bokeh tho it won't provide the more dramatic
separation from background that the longer focal length. The 60 is
all aces (and underrated) for portraits on APS-C bodies. Hope this
without complicating your decision