Upload & Sell: Off
As I think I posted in a previous 200-400 thread, back in September I was at an NFL game where an SI photographer was shooting with a 200-400. While I didn't get to handle it, I did quiz him briefly about it. His opinion was that it was sharp and that everyone will buy one. By "everyone" I interpret him to mean field sports photographers.
Therein I think lies a bit of an issue for Canon. This lens has the potential to cannibalize sales of the primes, particularly the 300 & 400/2.8. But at least based on what I've seen on the sidelines at sports events I've covered, most photographers have not made the switch to the latest super-teles. While these lenses offer some improvements, the image quality difference is likely not significant enough for many to justify the upgrade (clients will never notice the difference and aren't going to pay more in any case). At least for the sports photography market, this lens could be what Canon needs to realize a surge in super-tele sales. The downside will probably be poor resale value for those, like me, looking to unload previous version lenses due to oversupply on the used market.
While many here may seem to think the one-stop penalty is significant, the reality is the convenience of a zoom with a built-in teleconverter will likely outweigh this drawback for many sports photographers, especially for those with newer bodies capable of very good high ISO performance, even if the weight is the same or slightly more than the new 400/2.8. From my perspective, having recently moved to the FF 1DX after shooting APS-H since 2001, the biggest change in how I've been shooting sports is much more frequent use of the 1.4x TC on the 400. With the 1DIV, I rarely used it because I had the crop factor and pixel density to allow additional cropping, which was somewhat lost with the 1DX. But it has also been great to finally have 'real' 400mm back again for those times when the 520mm equivalent crop was too tight with ASP-H. While I could also shoot with a 600, it's a hassle to lug both around. And I hate using a teleconverter in poor weather conditions because of the probability of things getting wet and messing with the rain cover. To have a lens I'll never disconnect from the body during the entirety of an event and that replaces at least three prime lenses (300/400/600), is pretty significant. While f/4 is a slight tradeoff in background separation, I've yet to receive feedback from a client complaining of insufficient subject isolation. Just like the difference between the 70-200 2.8 vs. f/4 lenses, it's not going to be that much in practical use. Of greater influence will be the photographer's choice of camera to subject to background distances or whether the camera used is FF vs. APS-H vs. APS-C.