What Are The Weaknesses?
Where I find it lacking is being slightly soft wide open at 35mm. The softness is quite uniform over the entire frame and not significantly worse toward the edges or on one side. Some lenses tend to have areas near the edges that are quite severely softer than the center, an aspect that I find much worse than an overall softness. The focal lengths near the midpoint of it's range can be quite sharp, although I have not taken the time to analyze where the sweetness here lies. The situations where this lens comes to play are wide and varied and after all getting the shot is what counts. This lens can usually pick it off with quality that is well above the consumer grade zooms with this range.
At 350mm it suffers a little from softness toward the edges, however it is not drastic, with the best aperture being f/11 but still quite good at f/5.6.
The lens also vignettes quite a bit at 350mm. Vignetting at the long end however is many times not a problem because the subject is often at the center of the frame or has a background which obscures the problem.