I am using a 5d2 and I heard that the 600ex-rt's do not work with the 5d2 (sync speed).
I can't believe that. The 5d2's sync speed is 1/200, just like the 5d3.
Would you believe if you read it on p.51 of the 600EX-RT manual? With pre-2012 bodies, the 600EX-RT loses one stop off the max sync speed when triggered wirelessly with the RF system (e.g. 1/100 s max with the 5DII), and HSS and Group control are not possible via radio trigger, either.
Seriously? Wow. I guess I should have RTFM before I opened my big (proverbial) mouth on the internet. This actually shocks me. Anyone have any idea why it does that? I have a hard time believing it's nefarious, not something as important or odd as that.