The 600mm is a beast of a lens. The lens is over 11 pounds and must be cradled to walk comfortably with it. As it is a pricey lens, the best situation would be to place it on a tripod and gimbal as per my post of June 11, 2013.
I put the lens and lens hoods together and took several self-portraits of myself with the 600mm f/4 D ED-IF II to show just how large the 600mm lens is in comparison to my 5' 7" frame. I'm not tall and if held cradled in my arms vertically attached to my D300s, the lens reaches the top of my head.
Just tried the 600mm handheld today. The more I do it, the easier it gets, but I still would not recommend doing it for very long. And considering that most people shoot with their right hand and hold the lens with their left. It's like having an 11 pound exercise weight. Unless you want to look like Peewee Herman on your right arm and Hulk Hogan on your left. It's not suggested that this be done for very long stretches. It's only thanks to arm curling 30lb weights, that this 600mm handheld is as easy as it is and it's still pretty stinkin' difficult.
Again, do not think that this is a great idea to do... Handholding the Nikon AF-S 600mm f/4 D ED-IF II.
I'm sure the 600/4 will be quite the addition to the shooting arsenal. The 800mm is pretty much overkill on the finances. The 600/4 is stretching my finances by a considerable margin and only do-able thanks to a payment plan with Christian that we'll be working through until May 2014. As stated, the tripod is definitely necessary on this lens. I wouldn't advise anyone to handhold unless it was for a few seconds at a time.