The test shots below, cropped to 1000x1500 pixels, demonstrate "barrel droop" in this lens at 300mm extension and at apertures larger than f8.  Naturally, it's most noticable in photos with high contrast where horizontal edges are fuzzy but verticals remain sharp.  Change the camera orientation from landscape to portrait, and it's still the horizontal lines that are fuzzy.

For these sample images, the camera was mounted on a solid tripod, the mirror was locked up, and I used an electronic shutter release - all in order to rule out camera shake.  The results were the same, with or without Image Stabilization on.

Below the test images are three photos taken in the field of a Great Egret.  Despite very fast shutter speeds, you can see how horizontal and near horizontal borders between the bird and the background are fuzzy while near vertical borders are sharper.  (Note that the blurring is present in borders between less contrasting colors, but is not quite so noticable.)

Last year, I sent the lens out to Canon.  They replaced the IS and focusing motors but did not correct the problem.

 

 

Camera in Landscape Orientation: 1/250th, f10, ISO 100: Horizontal Edge Sharp

 

Camera in Landscape Orientation: 1/640th, f5.6, ISO 100: Horizontal Edge Fuzzy

 

Camera in Portrait Orientation: 1/100th, f11, ISO 100: Horizontal Edge Sharp

 

Camera in Portrait Orientation: 1/320th, f5.6, ISO 100: Horizontal Edge Fuzzy

 

1/500th, f5.6, ISO 200

 

1/1000th, f6.7, ISO 200

 

1/1000th, f5.6, ISO 200