I recently tried the Graslon Prodigy after reading good reviews. Yes, it does a good job softening the light but the thing is big + heavy on my 600EX/RT. The fancy connection system to mount on the flash , to me, isnt very secure. Won't take much of a "bump" to knock it off. As an example of the weight, if you tilt the head up to bounce, not that you would with this thing, the weight of the Prodigy is too much and the head falls back to 90 degrees.
I returned mine and now going to order the smaller version "the Insight". Hopefully its lighter and does a good a job as his bigger brother.