I had one for a very short period of time. Not very smooth, wouldn't lock down tight for carrying the lens/camera combo over the shoulder. Called Kirk and they said the fact that it wouldn't lock down was part of the design. When I sold it I listed all the things I didn't like about it and priced it accordingly and it sold.
I have one, works pretty well IMO. Not a Wimb. II but better than a Sidekick. I did not care for all the offset that a 600 has on it. Kept it for a backup or when I want two long lenses available.