Got one for my Canon 600MM F4 IS this summer. I am shocked at the improvements in the number of keepers. Shooting from a heavy carbon Gitzo, Wimberley gimble, and a cable release. After firing about 15,000 shots down range I estimate that keepers have conservatively improved by 30%. Shooting single frame, stopped down, high shutter speeds, improvements are probably minimal if any. Shooting wide open, low shutter speeds (down to 1/15 - that's right 1/15th with a 600mm with 1.4 extender) bursts from my 7d yield remarkable improvements. If you have that requirement, then this is a must have.
Shooting sustained continuous bursts generates vibrations that seem to put my heavy rig into a very slight shake no matter how I have tightened things down. Add the RRS Long Lens Support and the results are nothing short of remarkable. The transmission of the vibrations down the lens barrel are dramatically reduced. I have had the 600mm for 6 years and have used every technique/support and have never gotten results this good. Case closed.
Keith Bishop - Pro