I know that with two 600EX-RTs or one 600EX-RT + ST-E3-RT, that the off-camera flash unit has a button that can act as a remote shutter release.
My questions are:
1) Can you half-press the button to make the camera Auto-Focus, and then full-press the button to take the shot?
2) If you hold down the button, can you shoot in continuous burst?
Basically I'm trying to figure out if the button on the 600EX-RT acts exactly like the camera's shutter button.
1) NO YOU CANNOT. It is a one press deal, if you are in AF and it can't acquire focus it won't shoot, but it is not a two stage press like the conventional remotes. There are no half presses, focus and shutter are activated together, it doesn't work like a shutter button, you can't acquire focus without taking a picture.
2) NO YOU CANNOT. One push one picture (or not if the AF was on and it didn't acquire focus).
Effectively no, it does not act like a regular shutter button.
I use this feature a lot and there are a couple of quirks, a 600 EX-RT has to be the trigger, the one in your hand, the ST-E3-RT cannot do the triggering so must be on the camera.