Those of us who shoot birds know that often, you simply cannot have a long enough lens. That's why Art Morris routinely shoots with an 800/5.6L IS, often with a 1.4x TC, and why Bryan brought the 800/5.6 on his recent Florida trip.
But recently, Art mentioned that he'll be selling his 800/5.6L IS. Why? The 600mm f/4L IS II is an all-around better choice. Art posted his rationale yesterday in a blog post. Actually, nothing surprising there... It was clear as soon as the 600/4 II became available and Bryan posted the ISO 12233 charts that the new 600 II + 1.4xIII was sharper than the bare 800 (comparison), and that the 600 II + 2xIII was slightly sharper than the 800 + 1.4xIII (comparison). That meant that the new 600 II was effectively delivering a longer focal length, plus it's significantly lighter, and cheaper than the 800/5.6 - a win/win/win in my book.
In his review of the 600/4 II, Bryan questioned whether the 600 II makes the 800/5.6 obsolete...I think the answer is yes. I expect Canon knows that as well, and that at some point we'll see a new 800/5.6L IS II that incorporates the weight-saving improvements of the other MkII supertele lenses (remember that the original 600/4 IS was much heavier than the 800/5.6, so a new 800L would be lighter than the 600 II), and delivers better optical performance. That will mean the return of a more diffucult choice...better IQ is of more benefit than 5-7% longer focal length, and no more weight advantage...vs. the almost certainly much higher cost of an 800 II.
But until there's an 800/5.6L IS II available (not announced...but actually available, given the long delay from announcement to availability for the current Mk II superteles), the 600 II seems to be the clear choice for someone looking to get the most reach they can.