Top positive review
It's in a class of its own - very impressive
6 June 2017
I've been using Canon Speedlites for seven years now - this is the fifth one I've owned and this is the third model I've owned. As all my previous Speedlites are variants on the 430 theme this 600 EX II-RT gives an initial impression of being on the large and heavy side. As time goes by it seems not nearly so large while my 430s have begun to look rather small and it seems less heavy on my 5D.
In terms of performance I think the 600EX II-RT is in a class of its own, whether it's as my only flash*, or as a master for either the optical or the RT wireless systems, or as a slave in either the optical or the RT wireless systems - this flashgun does it all and does it all superbly. In situations where a 430 has reached the upper limit of its output (e.g. if you need to remain on low ISO) this flashgun will keep pumping out more light to get that exposure right - in terms of light produced this flashgun claims to be two thirds of a stop more powerful than a 430 - that's 67% more light at full power - so not quite twice as powerful as a 430, but still substantially more powerful.
In terms of build-quality I have long thought that Canon Speedlites were in a class of their own - and I have no reason to think differently of the 600EX II-RT - this is a quality piece of kit and I'm sure it will continue to perform superbly for many years. All-in-all I'm very impressed with this flashgun and I'm happy to class it as 'highly recommended.'
*The versatility of this flash is demonstrated by the following options - all of which I have tried successfully:
On-camera as (1) my only flash, (2) optical master, (3) radio master;
Off-camera attached by a shoe cord as (4) my only flash, (5) optical master, (6) radio master;
Off-camera as an optical slave unit controlled by (7) a Canon ST-E2 unit;
Off-camera as a radio slave unit controlled by (8) a Canon 430EX III-RT Speedlite, (9) a Yongnuo YN-E3-RT unit.
I think that the possibilities opened up by all these options makes this flash an impressively versatile machine in an impressively versatile system.