I'm making some changes in my bag in the next week or so and wanted to get a bit of input on what I know is a subject thats been talked to death! What I'm interested in is people that have shot motorsports or any other high speed sports with the non IS 70-200 2.8 and/or the IS F/4 70-200. I'm curious about how much more accurate the auto focus becomes with the center focal point being more sensitive with 2.8 lenses. I've shot with a 2.8 and haven't really noticed much if any difference. I'm very happy with my non IS F/4 but I'm going up to a 50D so I'm thinking the far better resolution of the IS F/4 may be a better way to go. What do you guys think? Thanks in advance for everyones help.