I have used a 100-400 with a 30d for African wildlife photography. I have been disappointed at focus at the long end in many instances, but have also had enough beautifully sharp images at various subject distances to be convinced that the glass itself is very capable. My suspicion is that the issue may be with the camera ... I think a 1dm3 would up the performance quite a bit - though losing some reach.
Incidentally, this lens is one reason I would struggle changing to Nikon - there appears to be nothing in its focal length and weight class to match it on the "dark side". Despite its issues (no dust sealing, somewhat awkward handling, and distracting bokeh in some situations), there is nothing else that can match it for focal length, zoom versatility, Image Stabilization and light weight (it's a relative term!). Often a 500f4/600f4/200-400f4 is just too bulky for quick handheld use. (Haven't tried a 400 f4 DO though - would be very interested to know how usable that is handheld)
My brand new 100-400 got its first look at light this past weekend at the Indianapolis Tennis Championship qualifying round. Unfortunately I erred by keeping the ISO at 400 and not dialing it down to 200, so there is a bit too much noise if I pixel peep on images without a lot of light. Here's an example of one where the lighting was strong and the image was very sharp. Jeff Tarango was one of the qualifying participants. I found out he's most well known for his temper, once being banned from Wimbledon for claiming one of the refs was corrupt
Down sampled image is first. 100% crop of that image is second. No other post-processing like USM.
Canon 450D, 100-400L @ 400-mm, ISO 400, 1/2000, f8.0.
