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I found PW Flex to be unstable - bought Phottix Odins instead. Half the price, better performance.
And, if you have other manual flash utnits you want to use/throw in the mix, you can mix and match with the cheaper Strato units. Phottix are on the 2.4Ghz frequnecy band which is less affected by Canon RF noise. Still haven't tried with the new Canon flash units, but would prefer to use the Canon RF system whenever possible. Think it will be hard to beat, with audio feedback and all. If you want to mix with other flash equipment then I would try the Phottix Strato which has a hotshoe that lets the signal through so you could probably place the Canon ST-E3 on top of the Strato transmitter and have the best of both worlds.