I was teaching a workshop today, so I didn't have a lot of time to photograph. But I used the 50D and the 500mm f/4 lens exclusively (with and without the 1.4x). I had no problems whatsoever with AF with that combination; fast and precise. I was easily locking focus on cranes after the sun had set, with light so low that at f/4 and ISO 3200 I was getting about 1/40 sec shutter speeds or slower.
The Cranes once again lifted off early this morning so I decided to work on pan-blurs in the less-than-optimal light. I like the wing position and open beak on this bird.
Canon 50D, 500mm, f/4, 1/400, ISO 1250, no flash, hand held
Amazing, Doug. Love that soft diffuse light with the post sunset tones. Open beak and sharp eye are perfect. Maybe a touch more NR on those right wing primaries.