There are times I think of selling my EF400 - just on the basis of age and lack of technology in the form of IS. I have IS on most of my lens, and i think it has a real advantage and sometimes a "placebo effect" too..
Had a day out in a nature reserve yesterday, and was mobile. The 400mm f/5.6 was the lens in use, and i'm always shocked at the results in terms of clarity.
Was framing a static shot in a dark pool, when a kingfisher came in. 1/400 isn't ideal for a hand-held shot of such a subject, but got a high percentage of keepers. Kingfisher (Alcedo atthis) through the gap.
, on Flickr