I think it still comes down to Canons very poor quality control for there L lenses.
I took me two exchanges to get a good 17-40 and the one i have is very nice indeed.
Has great sharpness at F4,good color and overall is very nice.
Needing the F2
.8 i have picked up a 16-35 with the intention of selling the 17-40.
However like many many others i have got a 16-35 which is very nice from F3.2-3.6 upwards but is soft as in the corners at 2.8.
Given that you buy this lens over the 17-40 for the 2.8 and its pretty impressive price you would get a product which is at least usable.
Like many others i am in the dissapointed 16-35 users group.
If you can find a good one they are awesome but from reading it seems like thats about 1 in 5 who atually do get a good one.