I have a 5Diii, yesterday I purchased a 580ex ii (considered the 600 but already have some wireless triggers and not sure there is a big enough difference to justify the extra $$$) (have to say the 580's seem to be disappearing quickly from retailers), have been using a 430 ex ii as a smaller walkaround for the 7D and 5Diii.
Was shooting a portrait yesterday evening using the 580 as a master (no light) and the 430 through an umberella.
For shits and giggles I turned off all the ambient lights, it was nearly pitch black, shooting in portrait with outer AF points, the AF light lit up the model like some Sci Fi movie and AF locked on (using 70-200 2.8 II approximately 6 metres from model)...
Maybe the 600 AF light is better, but in this situatuion I was pleased...