A few things:
The Gr mode doesn't allow more the minus one stop sync with older than 2012 cams. So if it's a 5D2 this means the sync is 1/100.
If you use a 5D2 or 7D you may use ratio mode ore no ratio to benefit from the HSS and sync.
The new RT system doesn't allow 2nd rear curtain (not sure if only in Gr mode).
Take those things with a grain of salt. Until we will get our hands on this is what I know.
The Gr mode is a 5 individual group controlled like Nikon's CLS system only that Canon's is a 5 radio groups. Each group has it's way independently: ETTL or M or off.
For those with studio strobes the 3rd party is the way to go since Canon doesn't provide radio for them.http://pixsylated.com/blog/canon-600ex-rt-first-impressions/comment-page-1/#comment-3572
Charleston Dave says:
March 2, 2012 at 2:20 pm
Congrats on being a leader, Syl! Not a bad subset of features compared to your Speedlite wishlist for Canon from a year or two ago!
The form factor of this new 600/STE3 system is similar to the Pocket Wizard Flex/Mini/AC3 alternative I use. All of the shoots you demo would be easily accomplished with the PW system.
From your video, the new 600/ST-E3 offers these advantages over the PW system:
1. A beep when the remote flash is ready (one beep from the ST-E3 when all flashes are ready? or a beep from each flash?)
2. Up to 5 groups (not 3 as in PW)
3. Fancier LCD displays
4. Claimed longer range, although I have used the Flex/Mini system reliably at 120′ and haven’t needed anything longer than that.
whereas the PW system offers these advantages:
1. Works with pre-600 flashes
2. Offers 32 trigger channels, not just 15
3. Works with other PW triggers, making it easy to combine studio strobes with Speedlites.
4. Offers a mechanical flash power dial for each individual group in E-TTL or manual, instead of a single touch-panel.
The biggest missing piece of the puzzle is an under-$100 device that plugs into pre-600 Speedlites and communicates with the new ST-E3 radio transmitter. Those of us who have invested heavily in earlier Speedlites will be reluctant to discard our investment. You reference a combined radio/optical triggering arrangement but I’m dubious about the complexity of a hybrid system.
I'll add having a 600EX-RT on the cam, you have AF assist lamp and radio Gr mode.
If you use PW on the cam with an EX on it, you miss the AC3.