Now that I’ve taken this out a couple of times, I’ll answer the question everyone asks: “why should I use the more expensive 800 f/5.6 instead of a 600 f/4 IS with a 1.4x converter?”

1. It’s a bit lighter and the IS is clearly better (4 stops) than the 600mm (2 stops) which means, in a pinch, I can handhold it for a shot or two, more like if I can brace myself on a fencepost, etc.
2. The 800 f/5.6 IS is sharper than the 600 f/4 with 1.4×.
3. The IS system does a better job of controlling mirror slap, allowing shots at 1/5 – 1/20th second (on a steady mount of course) that get blurry with the 600mm.
4. It’ll let you shoot 1600mm with a 2x TC, although you do have to manually focus.

Now that I’ve shot with the 800mm, I like it. I definitely prefer it over a 600mm with teleconverter if I know I need the focal length.

Addendum: Now the question has become “Why should I shoot the 800 f/5.6 instead of the lighter 600 f/4 IS II with a 1.4x teleconverter?” I don’t know. There’s something you never read on the internet: I don’t know.
April, 2013