I have so far always been handheld for all my long lenses - the day may come I'm unable to cope with it for long hours, but so far, knock on wood, so good. I don't use strap systems, cases, bags, or pods of any kind. I do usually mount pistol grips to the lens collars for extra stability and support, both when shooting and when carrying by the lens collar handle. I will alternate carrying the camera and lens upside down by the lens collar handle, alternating between left and right arms, and when it starts to get too tiring, I'll bring the camera up and cradle the lens across my left arm like I'm holding a baby. I like being able to quickly move the lens up to shoot at all times, in any direction, so handheld still works best for me with all lenses. I don't have anything as giant as a 600mm F4 - my lenses have included 400mm F4.5, 300mm F4, 200-500mm, 150-600mm, and 100-400mm lenses, with and without TCs. All have worked OK for handholding. I admit a 600mm F4 might wear on me a little sooner...so with a lens that heavy, I might consider hand holding for shorter periods of time, maybe bringing along some kind of sling or monopod when I'm on a long hike or walk for many hours with such a lens.