I bought a second hand 17-40 and it is soft on the left side most noticeably at the long (40mm) end even stopped down to F8/F11. I posted a few days back on this, but wasn't in a position to post images then, but now can.
I took the same view at 40mm and f8 with the camera held normally and the camera held upside down to test that there definitely is a softening on one side of the lens - shows up in the first shot and the second is sharp as this is the other side of lens (held camera upside down). These are only 50% crops so the problem can be easily seen without much blowing up. The softening extends a quarter of the way into the frame.
Is this really "normal" for this lens as some have suggested to me or would you regard this degree of de-centering as faulty?