@Steve & Keith:
I think you guys are conflating the purpose of having good equipment with who might potentially view your work down the road. As a photographer, we generally don't think: Oh, I can get away with using this crappy soft lens because no one who ever views my work would ever know. That's just inane.
Having a good lens is about eliminating the limits of your equipment, making YOU yourself the limit. I also still don't think you guys fully understand how good the Mark II supertele lenses are. The older 600/4 L lens produces visibly softer results than the 600/4 L II lens. It produced softer results than the 800/5.6, where as the new 600/4 L II produces sharper results than the 800/5.6 when used WITH the 1.4x TC III. You don't realize the differences until you've actually tried it. I thought my 100-400mm L was excellent...and it does produce sharp results...but as good as it is, it still doesn't compare to what the 600 can do...and at 840mm no less. Using this lens means that you, the photographer, become the limiting factor on resolving power.
As for whether the differences can be realized? Yes, they can. Absolutely! How often have you not been able to get as close as you need to fully maximize the pixels of your sensor at 300mm? At 420mm? Or even at 560mm? Even if you do fill the frame on an APS-C, does your IQ compare to that of a frame-filled FF?
I can use 1200mm f/8, with fast focus, on a full frame camera, and get away with shots that would normally scare birds off. Egrets and herons are jittery birds...you break their comfort bubble, they are gone. I could get this kind of framing with my 7D...but I could never get this kind of IQ...and certainly not from the distances I'm getting it at now. (These photos aren't the kind I usually share, the lighting on this day was TERRIBLE, resulting in high DR on my subjects, but they are some of the few I recently captured at 1200mm f/8, so just to demonstrate the point):
Thanks to having a very long lens that is actually able to perform AF reliably, quickly, and consistently at f/8, I was able to pack on the pixels in this shot. I did not have to get so close to the bird that it took flight in fear. I was actually able to stay at a very comfortable distance, which allowed the birds to maintain their natural behavior and comfort levels. And this is all at f/8...a diffraction limited lens is still resolving a ton of detail. The 7D AF at 840 f/5.6 also doesn't even perform as well as the 5D III AF at f/8.
So, I'm sorry, but you can't tell me that the differences are small. It's not just about IQ, although that's the key factor. It's about raw capability, it's about achieving things at a high level otherwise not achievable with lesser gear, it's about eliminating the limits your gear imposes on you, forcing you yourself to become the limitation (and that, my friends, is something eminently remediable. 
)
I also do believe that people can tell the difference. I had sharp results with my 100-400, and I always received compliments for that from people who see my work. However, I ALSO get compliments akin to "Wow! Your work was always sharp, but this is even more amazing!" since I've started using the 600. Your average viewer isn't oblivious to detail...they do notice it, and many can and do tell the difference, even if they don't know exactly what's causing it or where it's coming from, even when they are not photographers. Many also notice other differences...for example, I received some comments about my background blur when using the 100-400...however since switching to the 600, people notice more how clean and smooth the backgrounds usually are. Or, conversely, they recognize how isolated and enhanced the birds or wildlife are because of the creamy backgrounds (even if they don't recognize that its the difference in DOF and background blur that's causing them to notice the subject more), with comments like: "Wow! The birds just pop right out of your photographs now!"
I think you guys are conflating the purpose of having good equipment with who might potentially view your work down the road. As a photographer, we generally don't think: Oh, I can get away with using this crappy soft lens because no one who ever views my work would ever know. That's just inane.
Having a good lens is about eliminating the limits of your equipment, making YOU yourself the limit. I also still don't think you guys fully understand how good the Mark II supertele lenses are. The older 600/4 L lens produces visibly softer results than the 600/4 L II lens. It produced softer results than the 800/5.6, where as the new 600/4 L II produces sharper results than the 800/5.6 when used WITH the 1.4x TC III. You don't realize the differences until you've actually tried it. I thought my 100-400mm L was excellent...and it does produce sharp results...but as good as it is, it still doesn't compare to what the 600 can do...and at 840mm no less. Using this lens means that you, the photographer, become the limiting factor on resolving power.
As for whether the differences can be realized? Yes, they can. Absolutely! How often have you not been able to get as close as you need to fully maximize the pixels of your sensor at 300mm? At 420mm? Or even at 560mm? Even if you do fill the frame on an APS-C, does your IQ compare to that of a frame-filled FF?
I can use 1200mm f/8, with fast focus, on a full frame camera, and get away with shots that would normally scare birds off. Egrets and herons are jittery birds...you break their comfort bubble, they are gone. I could get this kind of framing with my 7D...but I could never get this kind of IQ...and certainly not from the distances I'm getting it at now. (These photos aren't the kind I usually share, the lighting on this day was TERRIBLE, resulting in high DR on my subjects, but they are some of the few I recently captured at 1200mm f/8, so just to demonstrate the point):
Thanks to having a very long lens that is actually able to perform AF reliably, quickly, and consistently at f/8, I was able to pack on the pixels in this shot. I did not have to get so close to the bird that it took flight in fear. I was actually able to stay at a very comfortable distance, which allowed the birds to maintain their natural behavior and comfort levels. And this is all at f/8...a diffraction limited lens is still resolving a ton of detail. The 7D AF at 840 f/5.6 also doesn't even perform as well as the 5D III AF at f/8.
So, I'm sorry, but you can't tell me that the differences are small.
It's not just about IQ, although that's the key factor. It's about raw capability, it's about achieving things at a high level otherwise not achievable with lesser gear, it's about eliminating the limits your gear imposes on you, forcing you yourself to become the limitation (and that, my friends, is something eminently remediable. ) I also do believe that people can tell the difference. I had sharp results with my 100-400, and I always received compliments for that from people who see my work. However, I ALSO get compliments akin to "Wow! Your work was always sharp, but this is even more amazing!" since I've started using the 600. Your average viewer isn't oblivious to detail...they do notice it, and many can and do tell the difference, even if they don't know exactly what's causing it or where it's coming from, even when they are not photographers. Many also notice other differences...for example, I received some comments about my background blur when using the 100-400...however since switching to the 600, people notice more how clean and smooth the backgrounds usually are. Or, conversely, they recognize how isolated and enhanced the birds or wildlife are because of the creamy backgrounds (even if they don't recognize that its the difference in DOF and background blur that's causing them to notice the subject more), with comments like: "Wow! The birds just pop right out of your photographs now!"
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I also don't think it will matter how sharp Canon may make any 100-400 II...the difference in pixels on subject is really going to create a divide between those who use the 150-600's and those who stick with the 100-400 range (or 80-400 on the Nikon side).
I did not actually pay full price for my 600...I got it off a Canadian site for $10,800 (and that was back when the list price was still $12,999). Still, $2500 for a lens that natively reaches 800mm f/5.6 is really freakin good.
