Sometimes you have to hand it to Canon for the effort in making some unique auto focus lenses. No one I know makes an auto focus 100mm f2 lens (the 100mm f2.8 macro lenses don't count). The other unique auto focus lenses not available from other manufacturers are: 85mm f1.2, 200mm f1.8, MP-E 65 macro lenses that has a magnification ratio of up to 5x, and finally, let's not forget the monstrous 1200mm f5.6.
The Canon EF 100mm f2.0 lens is a neglected lens. Many people who are looking for a portrait lens almost invariably choose the slightly faster sister lens, the EF 85mm f1.8. I choose the path less taken, so to speak, and bought the 100mm f2 instead.
I can't tell you how happy I am with this lens. Talk to anyone who owns this lens, and they will tell you how good this lens is. It's compact, light weight, fast (both focusing and brightness), very nice bokeh, and very sharp with excellent colour and contrast. I have had it since 2003 when I got Digital Rebel 300D. It was my favourite lens until I got the 85mm f1.2L. But, even then, I take it out once in a while, to re-kindle the original love I had for this lens.
What many people may say about this lens is that it's not very sharp wide open. I say these people either got a bad copy, the camera has focus issues, or are not using it properly. This lens is very sharp wide open at f2. I did notice that a better body gets more keepers. On the 5D and the 1-series of Canon bodies, this lens performs brilliantly wide open. Even on my Rebel 550D (T2i), it was very good at f2.
There is very little not to like about this lens. Very well built with ring-type USM motor for accurate and silent focusing that also offers Full Time Manual (FTM) focus adjustment, which means you can adjust focus by hand at any time, unlike the Arc Form Drive lenses. If I have nick pick, it's a bit long indoors, especially on crop bodies. Be sure to get the hood for this lens. It makes the lens look good, and helps to protect the lens if you drop it hood down.
My friends had asked me why I still keep this lens when I already have the 85mm f1.2L. I am keeping it because it's a great lens. A lens I enjoy using when I don't need the f1.2 aperture.
Megan - Canon 300D & EF 100mm f2 @ f2.8. Larger Picture.
Little Angel -- Canon T2i (550D) & EF 100mm f2 @ f2.