I've had this exact same combo for a while, although I haven't really used it much in anger. In anything except for the brightest light the AF is awfully slow and often hunts. I'm not very impressed by my 400D's (Rebel in US terms?) AF in general but I have a 5DmkII on order so I'm hoping that will improve things somewhat.


It's just a stopgap measure for me though, albeit one for the forseable future. All the advice I've had is that it doesn't compare with the 400mm f5.6 for AF speed etc. But that doesn't have IS - my 70-200 f2.8 does. If I'm to spend £1k on another lens I want it to have IS - I find it really odd that the f5.6 varient isn't available with this, and wonder if Canon will release one at some point.


If I do find myself with money to burn - and if I either decide that IS isn't so important, or Canon release such a beauty at f5.6 - I will probably get one. But the 2x Extender still won't have been wasted - I can then use it for a lovely 800mm zoom, albeit with MF only. It might not be much use but it would be cool to have .