It depends on what kind of macro you like. The 100mm is great for insects while a shorter focal length may be more useful for other situations. None of the Canon-made lenses has any real disadvantages except the price.
I currently have the Tamron 60mm/2 DiII which is also made for APS-size sensors, but only shows vignetting when focused to large distances. It's great for it's price, but not ideal for 35mm film cameras since it doesn't go to 1:1 with the larger negative - not the right thing for taking pictures of insects' eyes, but good enough for flowers or tabletop macros. It's main advantage is the big aperture and great sharpness from f/3,5 upwards. My personal recommondation for everyone who uses both the digital and analog EOS-system.
Also, Yay for Elan II(e)! Love the camera