I have the RRS MC-34 and MH-02 Pro head, both are excellent. The combo easily supports my 600/4L IS II, but I also use it sometimes with other lenses (which is why I got the MH-02 vs. the -01, the -02 clamp can be rotated w/o tools).
The monopod and head are very well constructed (as is all RRS gear).
Personally, I chose the screw clamp over the lever clamp because I have a (slight) concern that I'd inadvertantly open the clamp while carrying the 600 II by the monopod resting on my shoulder, catching the lever on a backpack strap, for example. I do have their lever clamp on my PG-02 side mount gimbal (and the leveling base clamp is a lever as well), and they're quite nice.