At evening events my 1D MkIV is pretty hopeless when it comes to low light AF. The AF assist performance on the 600EX-RT is modest to say the least when used with the 70-200 f/2.8isII. It needed some help...
So my two Yongnuo YN-622C have been my go-to accessory for AF Assist for the past few years (2012). With time they have become a bit knocked around and recently erratic. Batteries are charged, all terminals are clean and factory reset performed OFTEN. Anyone with YN-622C's will know the pattern thrown by the AF Assist is pretty weird, but it's been the best we've got.
Is there an updated alternative to the YN-622C? All I use it for is for AF Assist, when I need triggers I use the amazing Phottix Odins. The YN-622C pass-through design is what makes it so useful to me as I sit the 600 EX-RT on top of it. Yongnuo does make a standalone hotshoe-mount AF Assist http://flashhavoc.com/yongnuo-yn12af-af-assistant/
but it's not a pass-through design.
A web search has not come up with a newer alternative to the YN-622C, any suggestions would be greatly appreciated. If not, I'll just get a couple more YN-622C's.