yeah, I believe I was a pre-addict after using my 70-200 in the backyard w/ birds, and officially after taking my 300 out for it's first spin.
I opted for the 600f4 just since I'm holding onto my 300f2.8, it made more sense than a 500f4 to me, if I sold the 300f2.8 like you ended up doing, then I'd totally go with the 500f4 over the 600f4, absolutely...
but as it is basically with various converters I'll have a 300 f2.8, 420 f4 w/1.4x, 510 f4.8 w/1.7x, 600 f4, 840 f5.6 w/1.4x, and 1020 f6.7 w/1.7x....before any cropping on my d800. Though I don't have a 1.4x yet, I'll get one in the future.
that way I can have the 600f4 + 1.4x on a gimbal/tripod. and the 300f2.8 either bare or with the 1.7x close by as needed...unfortunately I had to sell my D3s to get the 600f4, so I'll need another body sometime for backup too, in which case I'd have the backup on the 300f2.8 (which may be an 800e, or maybe a 7100 or something else), and the 800 on the 600f4, that is if I'm setup in a blind or a staking out a spot...if I'm out hiking alot it'll probably be just the 300 + 1.4x or 1.7x, which I've been pretty happy with the results from.