I originally bought this lens for use with an Elan II film SLR back in 2003. It was extraordinary. The bokeh, focal length, and extreme background blur made it my favorite lens.
Fast forward to 2005. I bought a Canon Digital Rebel. Now my 100/2 was a 160/2. That's too long for indoor portraits and too short to be a real telephoto. It also had a lot more depth of field at the same angle of view, and thus noticeably less background blur. Slowly, I noticed that I wasn't using it as often, or really at all. So I sold it and bought a 50/1.4.
That's the unfortunate thing with this lens. As an optic, it's excellent. Fairly sharp from f/2, good contrast, fast and silent AF, and only moderate color fringing. But the focal length is just *so* inconvenient. This 100/2 feels cramped for space indoors, but too short to get up close and personal outdoors. The optical design is also optimized for close distances, so if you try to use it for distant objects at larger apertures like a real telephoto, fine detail disappears. It's also incompatible with Canon teleconverters, though you wouldn't really want to attach one anyway.
What I used this lens for on my Elan II was flattering women. It's extremely good at that. My 50/1.4 serves a similar purpose now, but at an 80mm equivalent, it's a bit on the short side. The best option would be a 70/1.4, but alas, that doesn't exist.
If you're considering this lens for a body with a 1.6X cropped sensor (Rebels, 10D-60D, 7D), consider the 85/1.8 instead. They're similar designs, but the 135mm equivalent is just inside the standard portrait range, and you may like it better.