From Syl Arena's Blog: Click here to read his full review

Sync Speeds — I shot at all sync speeds, up to 1/8000″ in High-Speed Sync, on both my 5DM2 and my 60D. I used both a 600EX-RT Speedlite and the ST-E3-RT transmitter to trigger. In hundreds of frames over the past week, I have not had one sync issue. The manual says that on pre-2012 cameras (anything other than 1D X and 5DM3), the system takes a one-stop hit in sync speed and that HSS is not possible. This will be an interesting area to watch as others begin to use the gear.