Rented one of these for RIAT this weekend...
I HATE it!
Seriously, For Airshows i've normally used my 70-200 f/2.8. THis used to be on my 40d and I upgraded to a 5dMkIII last year, the lack of crop on it made it difficluty to keep the aircraft on the focus spots I was after (not a kit issue, they were just too damn tiny), so for this year I decided to rent a longer lens.
Before my hit percentage was about 30% of the shots i'd be happy with (I wouldn't keep them all, but they were all correctly focused and exposed etc). If I've take 2k shots that woudl leave me with 600 or so to sort through (keeping 150 'ish for editing)
With this lens and my 1.4x II and 5DmkIII I reckon my hit percentage was around 80% and I took 4.5k shots! ARRRGHHHHHH! It's too bloody good!
Seriously though, it's an amazing bit of kit... Tested it out on Sunday with some Cricket shots and did some Macro with it on Friday and it's simply stunning.
Tempted to trade in my 70-200 f/2.8, but I know i'd miss the 2.8.... plus it looks different to the 70-200 and the Mrs would spot it straight away if it somehow managed to magically appear in the house again.....