Well, after reading this and other forums I was decided but also a bit concerned about the Quality Control from Canon.
Fortunatelly, I live now in China, so here if you spend 1000 bucks on a lens (which is what the shop attendant will make in 2 years) you are allowed to hang around and bother the shop far beyond the point where in US or Europe they would "send you home".
I took a test paper, hang it into the shop wall, and set up a tripod, my 30D, and tested 5 brand new 70-200 f4 L IS that I unsealed myself for more than 3 hours.
2 of them looked ok but had defects (1 had a strange defect in the lens close to the camera, like a coating problem, the other one had dust inside the outer lens).
From the other 3 there was a clear Best: the other 2 were respectively soft at 70-135 and 200 mm.
The one I picked was simply amazingly sharp at all ranges, and I could prove it consistantly in the test.
I shoot with a good tripod (from the shop :-), remote switch, mirror locked-up, 3 focal distances (70-135-200) and 5 shoots per focal distance, with and w/o IS (although the IS "test on a tripod" turned out irrelevant for obvious reasons) . No room for missinterpretation, same camara, light, person, day, target test paper, distance, simply all taken care.
I even code the names so that I would not know which lens shoot what photos to avoid a placebo effect.
I could test in a very well controlled environmen and just can tell you that even if all the lens were kind of OK, there are clear differences, and some are good, some are awesome.
I agree with the people that was not happy to pay 1000 bucks and needing to change the lens several times.
Maybe after this report more people will ask Canon to test lens before buying, but somehow they put themselves into this, isn't it?