I just purchased a Canon 100-400L last week and took it, attached to a Canon 7D, out this weekend for the first time to my son's baseball game. I attached a higher end Tiffen UV filter and a circular polarized of similar quality and was bitterly disappointed by the results- every photo was not sharp at all. See attached photo shot at 250 mm, F6.3, shutter at 1/160 and ISO 640. The attached photo is a good represenative of all the photos I took. I took the setup home, took off the circular polarizer and shot photographs of tall grass at a distance with only the UV filter on (shooting tall grass is a good method of checking for sharpness). I was simliarly disappointed if not totally aggravated with the obvious lack of sharpness with the lens open to 6.3 and shutter speeds above 1/500. My next step, when it stops raining, is to place the camera on a tripod, and shoot the tall grass with and without the UV filter. Hopefully, it is only a filter issue. It would be inexcusable for Canon to ship pricey "L" lenses that are not tested and determined to be completely sharp before leaving the factory. Note- the Canon 7D takes sharp photos with my Sigma 70-200 f2.8, OS. I Will keep the thread advised.
1. You need a much faster shutter speed than 1/160sec for this lens. I generally try to stay at 1/1000sec on my 7D, and will slow down the shutter from that starting point only if necessary. The IS system on this lens is older, and only provides 1-2 stops of image stabilization.
2. This lens is known to have problems when using filters. Keep the UV filter off. Also, the CPL filter will reduce your exposure by 1-2 stops, which will cut into your shutter speed. Keep the CPL off as well.
The 100-400L is capable of very sharp images, but you need to learn how to use it properly.