This lens has really made a difference on my 7D. AF has really made a difference. It is very accurate and BIF with the 1.4 is absolutely great and shaaaarp. I decided on this lens because I did not think the new 500 f4L would be a great upgrade. But I needed a good 300-420 lens for a walk around and BIF lens. It has surpassed my expectations.
In my opinion it is as sharp as the 500mm F4L IS with and without a 1.4 T.C.
Click on the photo above. You can see how much detail is there. This is with a 1.4 T.C. hand held. The new IS system is very good indeed. Both optics and IS make for a winning birding combo for me.
Go to my flickr site for more pics.