Here's another one with the 100-400mmL. This shot was taken on a tripod and it took several attempts to get a sharply focused image. This is a nearly 100% crop also and the subject must have been 100 meters or more away. It was also breezy. I used live view, magnified the image 10x, and manually focused to get this one. Luckily, there was lots of light.
400mm, f/10, 1/1000, ISO 400
This shot of several otters - it's hard to see them all - was taken hand-held with my new EF 100-400mm f/4.5-5.6L lens. This is nearly a 100% crop and you can see that it is pretty sharp. So, even though the IS system is not the most recent, it did a fine job. I have to raise the ISO and shutter speeds more than I like to maintain sharp IQ, especially on small, long-distance subjects like birds and other wildlife.
400mm, f/10, 1/640, ISO 400