I was using the EF-s 60mm Macro before purchasing the 100mm and found that I was able to keep my distance with the 100mm Macro lens, which is a good thing especially if I'm photographing a bug or spider..............
Full Time Manual focusing is enabled and the front element does not rotate, the lens is very quick to focus, and its very sharp even wide open, I found it can hunt a little when right at the end of its focal length, but this is more than likely me trying to get too near, the lens allows a minimum working distance of 6" from the end of the lens (without the hood in place, which is optional ).
So as a full time "macro" lens I favour the 100mm, but it can also be used successfully as a portrait lens, and as it has 8 aperture blades, can produce a nice background blur or bokeh.
The EF-S 60mm for me combined with the 50D is probably a more practical lens, as I tend to take more portrait / product shots but as an out and out Macro lens, the 100mm must be the best value and fun per pound......................
-more especially if combined with a full frame camera such as the 5D MK II.
A warning though - you will find yourself looking for tiny things to photo - No bug is safe. And with macro photography, you will find your neck/back hurting from looking down or being low down all the time!