I've got a Canon 580EX (original, not the II) flash, and now I'm interested in adding a slave flash. I stopped by a camera store this evening to look at the 430EX II, but the salesman advised me that:
an important difference between the xxxEX and xxxEX II flashes is that the latter uses a new communication protocol
due to the previous point, the 580EX won't reliably trigger a xxxEX II slave; it'll probably work some of the time, but it won't be dependable
ETTL only works with multiple 600EX RT units; lesser units don't communicate about exposure
most photographers use Pocket Wizards specifically because they preserve ETTL
All this came as quite a shock as I've always understood that a big advantage of Canon's system is that the body and Speedlite work together, etc. It seems unlikely that Canon would update their line in a way that's not backward compatible. So I went home and reviewed the manuals for the various units. It's pretty clear that the guy simply doesn't know what he's talking about on the third and fourth points; all the Speedlites seem to support wireless ETTL-II. But what about the first two points -- can I expect my 580EX to work well as a master with a 430EX II slave?