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Smooth, well damped operation of the zoom and focus rings. Should mount on the body with minimal play (there can be a little rotation, but of course no perceptible pitch or yaw). AF should be direct, accurate, and very quiet.
Lens creep refers to zooming a bit when pointed up or down, and won't be a factor with the 16-35, as it zooms internally, i.e., the front element doesn't move and the lens body doesn't lengthen. I don't own the 17-40 and am not sure if that is a similar design; probably is.