For starters, having the grip on your wpn is a day and night difference. After x number of hours on patrol, or humping through grapefields/ wadi's, fatigue of the forward arm (wrist) is almost non existant compared to the wpn without it. As for durability, in my experience this piece of kit will take care of you as long as you are decent with it. A drop of clp on the clamp lock and clamp itself everynow and then does wonders. The $300 price tag was a little detrimental to the piggy bank but worth every cent.
WRT positive control of the weapon and attaining sight picture/ snap shooting, the grip performs just as well as having a C9 grip on a regular C7. In my experience having the clamp locked on the 3rd ring from the breach provides the best position for operation and depending on whether you have a surefire, paq 4, etc... on the wpn, it provides a good point for the grip as the relative centre of gravity of the wpn, (once again depending on attachments to the wpn, and personal preference).
Having myself and several friends who had them on their wpn's during 3-06 I have had nothing but positive feedback and opinions WRT to this piece of kit, and hopefully if the trials pan out in Gagetown, we may or may not see it in the system in a few years time.
Hope this helps.
Fraz