A 600mm f/4 lens is one heck of a monster, all six and a half kiloes of it. However, once onto the tripod and well-balanced on the Wimberley Sidekick, it's fairly easy to operate. The only times I'm reminded of its physical size are when taking the eye away from the viewfinder, and when the AF occasionally start hunting. It doesn't do that often, though.
Here's one result from yesterday afternoon:
Pentax K10D, FA* 600mm f/4, sturdy tripod with ballhead and Sidekick.
1/3000s, f/9.5, ISO 800.