After my experience with even Canon Grips flexing with a 70-200mm f/2.8, I've just lept away from them. Having your camera mounted to a heavy tripod with a expensive head and then have a spring between them was very frustrating. I'd be interested in hearing how stable it is and if it droops with the new Canon Grip. I'm also wondering if they inproved the attach mechanism where many users have had stripped threads.
The Canon grip doesn't flex all that much - though it is felt when attaching a heavy lens. But then whenever I use heavy lenses and the grip together, I mounting the setup using the tripod collar on the lens instead of the camera so flex really isn't an issue (for me at least).