For all of you birders, here is a pretty amazing review of the lens by a guy comparing it to the 600mm f/4L IS (original version):
The comparison images of the Red-Tailed Hawk have the bird occupying half the height of the frame. At that size, the image should be extremely sharp with fine details of the plumage visible. You can see that the head of the hawk from the Tamron is soft. All of the images I have seen at 600mm f/6.3 are soft to varying degrees, as you would expect from the measured MTF charts published by different reviewers. However, the lens gets significantly sharper at f/8-f/11 and the lens should be very sharp. So would someone please post some shots at f/8-f/11 so we can see what the lens is really capable of doing. How about it Don?
Alan, I get what you are saying, but as Roger rightly pointed out in his comparison, it is the lighting and conditions that are going to make the biggest difference. I know what you want to see, but for many of us that have had the chance to use the lens, we simply don't have the kind of lighting conditions that are going to produce the most optimum photos. It's January, not exactly a month cherished by photographers around the world.
The takeaway for me is that here the lens was compared to a high end 600mm and wasn't trounced and that he got a great shot tracking with the lens.