I have had the 100-400l for 2 years and its only been in the last few months I have really taken advantage of it. It's a bit big for shooting at the race track I generally go to; flat with no grands stands or high points. But I shot the Transit of Venus with it, and went on a zoo trip, so I I think it will be kept in the stable for the mean time. I really want to hit up an airshow.
I bought a 70-200 F4 non-is, used, on Friday, so I have not had a chance to really use it. I will try and shoot with it this week, to get a feel for it.
This set up seems like a good combo, at 70, the 70-200 can basically stay on the camera most of the time. The 100-400 can be used if I need a bit more focal length. I would say get the 70-200 first, then get the 100-400 if you really need it. Or maybe get a crop sensor camera as a back up, gives you a 112-320.
