I've highlighted one of its short comings.
And it is hit and miss with ETTL, because of 18% grey metering, it won't fire that much a flash behind the subject, because of the whole black lit thing. It is actually very difficult. It is much easier to have the flash off camera going off a set value in manual and ETTL the front.
So I set 2 580ExII's up on manual with 2 flex TT5 receivers, then on camera I have a MiniTT1 with another 580EXII on top on EETL.
Camera set to <1/20th - f.8ish - 800+ISO - 2nd Curtain sync - and every night shot will be perfect......
AC3 seems like a waste of money then really, is there anything that I can replace it with that will still fire the Flex's off 2nd curtain? And give me the ability to change the 2 manual flash powers without walking over to them?