Thanks HD. I briefly owned the 100-400 but got a bad copy and returned it. I ended up with the 70-300L and really love its combo of great IQ, great IS and great zoom range. It spends a lot of time on my camera outside of the winter months. But those birds....

I've come close to pulling the trigger on the 400 f/5.6 a few times. The lack of IS has been the issue. It's not 500mm, but it might be the best option....