I put together a 70-200 f/2.8L II + 2xIII + 500D combo, and it's not too bad.
The AF does work, but it's herky jerky, and honestly it's better if you set it to the min. focus, and lean in and out until you nail the focus instead.
The big problem I have is that the DOF is TINY TINY TINY, but this is a general problem to overcome with any macro. Amazingly, stopping down really high didn't seem to hurt image quality that much.
The attached picture was taken with a 5D2 + 70-200 f/2.8L II + 2xIII + 500D + 580EX II... 6400 ISO, 1/200 shutter, f/64 (yes, f/64, not a typo).
The flash was bouncing on a high ceiling with the catchlight panel up for a small amount of direct light, and the shot was hand-held.
I had to boost the saturation/clarity/NR and darken the blacks to hide the horizontal noise bands... a 5D3 would be a much better camera for this I believe.