If any of you ever come to Orange County, CA, come to Bolsa Chica Ecological Reserve in Huntington Beach, you'll see about 20 600s and 500s both from Nikon and Canon linning up the bridge on a Saturday morning.
I personally like the 500 f/4because it's handholdable, not enough reach just pop in the 1.4x and it becomes 700mm f/5.6. The 600 is too long and heavy to handhold. I personally know 5 people that exclusively handhold their 500mm.