You are right. In terms of having a real apples to apples comparison I would have to take the same shots with two different lenses. That's something I haven't done. So it's not really a quantitative statement. What I did see in the past (different lenses, different photos) that the area of best focus is extremely crisp and sharp and then out of focus the details are blurred in a way that doesn't seem pleasant. Example here:


http://kubm.smugmug.com/Events-Whats-New/2009/Oct-09-Mamady-Keita-in-Phoenix/10046246_3x9t8#688445387_3Sw3F


This is a photo that a friend of mine took, who I adore for his work. On this particular photo I personally don't like the way the hair looks. Some curls are crisp and others are simply mush. Again, I'm aware that this is not quantitative and prone to personal and subjective liking and disliking. There are different kinds of bokeh and as far as I know there is no quantitative theory available to parametrize the beauty of bokeh.


That was my point to begin with. If I'd judge the lens by its MTF and aberrations, chromatic errors, I probably wouldn't like it. If I look at the photos, I like the results, so I like the lens. Very subjective....