Yesterday, I rented the 100-400 to shoot Santa Barbara City College v. Ventura College softball with my 40D. I was a bit worried about the lens being 'soft', so I wanted to see for myself. I rented from Sammy's. The lens I got was heavily used and abused. I found that the lens is fairly sharp wide open, but at 300-400 mm needs speeds in excess of 1/800. I found the 4.0-5.6 limiting, but not a complete deterrant. This lens did make me miss my 2.8! Overall, for bright sunny days, this lens does well, produces fairly sharp images, and is lighter than my 70-200 2.8 IS. I had IS on the whole time, and it gulped down 1 battery after one game of shooting! It focuses fairly quickly, but not as quickly as my 70-200 2.8 IS. My usual keeper rate with the 70-200 2.8 IS is about 90%. With the 100-400, the keeper rate dropped to approximately 75% - partly because I needed faster shutter speeds and got camera shake, and partly because the focus either missed or wasn't fast enough. The push pull is a pain, and I didn't like it. However, I would imagine that once you get used to it, it wouldn't be so bad.
My partner also rented the Nikon 200-400 4.0 for his Nikon 2Dx. Granted, the Nikon 200-400 is $5,000 whereas the Canon 100-400 is about $1,500. However, that lens is TACK SHARP and fast! Being that I shoot mostly outdoor sports during the day, that would probably be my lens of choice.
An image shot by the 100-400 with my 40D (ISO 200, F 5.6, 1/800, 250mm):

!
I got some nice images and it was reasonably sharp, I just couldn't get used to the push pull action.

