I have shown these comparisons a number of times. These are 100% crops. They correspond to 60 inch wide prints
IMAGE LINK: http://s37.photobucket.com …ison_zps4bd2a96f.jpg.html
The comparison is of the EF 24-70 MkII with the EF18-200 both mounted on the 60D (the only body I had available at the time).
For those of you that run Firefox or Chrome (I don't know whether IE works the same way) right click the image and Click view image. You get the same image with nothing else the tab. Now press CTRL and roll the scroll wheel of the mouse until the image becomes half the size. That corresponds to a 20x30. Chances are about 50/50 that one could identify the wrong lens (i.e. the same probability as picking up the sharper lens without looking at the photo).
Now, does it mean that the super expensive L lenses are useless, or an expensive luxury?
You are not going to see me running to the For Sale section before the word is out...
Certainly cropping is a consideration.
Subject isolation is another. What sold me to the EF 70-200 MkII was a photo of a dog team pulling a sled with the leading dog's face being in perfect focus and everything else fading into a blur. Glorious photo. I can try six ways from Sunday (and trust me I have) the EFS18-200 at 200 &f5.6 doesn't get you the subject isolation.
Certainly the 18-200's IS is not comparable to the 70-200 either.
IMAGE LINK: http://s37.photobucket.com …/70200_18200comp.jpg.html
At pixel peeping level the rendering and contrast of the 70-200 are unmistakeable. The lighting in this photo doesn't show it. Sharpnesswise, you tell me...
At over 2 hours shooting engagement the results of the EF 70-200's weight are unmistakeable also...

I feel that each lens in my collection has its place, actually the only candidate for departure may be the nifty (I may keep it just for kicks) if the new Sigma 50 Art proves worthy of its promise and a bargain in price.