I bought this lens a few months a go together with a 5D Mk111 body. I am convinced that there is a problem with the image stabilizer that causes image blur when I set the aperture to F5 or above. I have taken the lens to Canon 3 times and despite me demonstrating the problem they don't seem able to fix it. I know that a shutter speed of 1/10 sec is a bit slow for hand held but the comparison I made was between 2 photos both taken at that speed. I would be very interested to here from others who have this problem. Here is the test I did to demonstrate the problem.
1. Set the camera to 1/10sec at F5 or above. Set the lens to max zoom.
2. Take the first picture normally using a static subject with some clearly defined edges (such as text).
3. Take the second picture by first focusing with a half press, then press and HOLD the depth of field preview button to close the aperture, then take the photo.
With my lens, the first picture is blurred and the second is completely clear. This suggests to me that the action of closing the aperture when the photo is taken is causing the image stabilizer to lose lock. If the aperture is closed before the photo is taken (or if it is set to F4), the image is clear.
I have tried this lens on another body and get the same results. I also have a less expensive EF-S 15-85mm lens on a 60D and its stabilzer performs perfectly under these conditions.
Hopefully you can see the attached samples.