In general 300mm and 400mm f2.8s are for sports first, wildlife 2nd.
500, 600, 800 are for wildlife first, and sports/other applications 2nd.
I have a 400mm f2.8 IS (the first one) and a 300mm f4, plus both vIII extenders. This gets me a wide range of telephoto options, from 300mm to 1280 (equiv. on the 7D)
The 600mm f4 II looks astonishing, but waaaay past my budjet.
I actually like the 300/2.8 II for wildlife, especially deer and other ungulates. During this time of year especially, the males seem pretty docile. With as little as tan pants and a tan/light greenish camo shirt, they seem fearless enough to let me get well within 300mm range...even as close as 100mm range for a head shot. The weight and balance of the 300mm is a lot better, IMO, for your average walk-around wildlife photography than the 600mm (not that the 600mm isn't a great lens for that as well when you have more stationary or dangerous wildlife subjects.)
I agree, and you can stalk football players, baseball players, track and field athletes as well. You don't even have to worry about camo.
300mm easier to carry than 600mm? Now that's just crazy.