much has been said however lots of information seems to be missing.
So firstly, I have been in photography (D-SLR) for about 6 years having owned 30D; 5D and now 5D mkII. I have shot up to 10 weddings already and I have 2 small incredibly energetic kids so I have really some experience with portraiture photography so far. I do not want to say I am the best photographer (still long way ahead of me) but I truly know how to operate my camera.
As it was requested already and I fully agree, it is much easier to once see the pics than just blindly talking about them.
So to sum it up one more time. According to the counter of my camera which is currently slightly above 5K shots, I haven't experienced any focusing problems of above mentioned impact until about 4K shots. During my last Saturday wedding I took roughly above 5 hundred shots with canon 24-70 f/2.8L lens and almost 300 shots with Canon 70-200 f/2.8L IS mkII. The rest (about 140 shots) with canon 50 f/1.8 and canon 85 f/1.8.
I truly do not find 10 pin sharp pictures out of all the pics.
So I decided to take my camera with 70-200 lens out yesterday to test it. I am attaching pics that were taken 3 months ago when my 70-200 lens arrived vs. the ones taken yesterday. Important thing to note here is that yesterday I did not recompose! All the time using center point for focusing, focusing with thumb and firing right after focus was achieved whereas older pictures where recomposed after focus was achieved. As can be seen results are drastically worse now.
Please click on the image so you can explore it in 100% magnification:
OK focus no.1 ISO400, 185mm, f/2.8, 1/50sec!
OK focus no.2 ISO800, 200mm, f/2.8, 1/125sec
OK focus no.3 ISO100, 190mm, f/2.8, 1/200sec
and now pics from yesterday (these pretty much copy the quality of the pics taken last Saturday when shooting friend's wedding)
Bad focus no.1 ISO800, 200mm, f/2.8, 1/80sec
Bad focus no.2 ISO1600, 200mm, f/2.8, 1/125sec
Bad focus no.3 ISO1600, 200mm, f/2.8, 1/125sec
Bad focus no.4 ISO1600, 200mm, f/2.8, 1/125sec
Moreover I have an impression that this camera cannot rely cope with ISO higher than 800. It is either down to my focusing issue or high ISO adds to the problem even more.