What's next? 600-rt to replace the 580 ex ii s?
Depends on how you shoot. The 600EX-RT is a great flash, of course, but the main reason to get it is for the RF capability. UPS is delivering my second 600EX-RT and an ST-E3-RT to me today.
I sometimes shoot with only Speedlites, sometimes also add a monolight. I've been using PocketWizards for triggering because they can fire the blended setup (I had a pair of 430EX II's that I just sold). But the simplicity of the Canon-only system is attractive when using just Speedlites. I plan to try it with the monolight optically-slaved, and if that works for the blended system, great!