I am an ex Nikon user and a professional photographer and have the 90, 16, 23, 35 and 50-140 zoom lenses.
I have to say that the 90mm is the sharpest lens of the bunch, by a long way too.
I love this lens, when I look at the images I want to stop using all the other lenses, its so good. I may have been lucky and bought an excellent copy, this is one lens I value above all the others.
If it has a problem, its without OIS, you need to shoot hand held at 1/500 minimum for the best IQ. Wide open its excellent, f2.8 to f5.6 it is outstanding.
When the new XH-1 with IBIS arrives, that should make this lens the greatest ever,
I am only hoping that IBIS does not have any adverse effect on the IQ of this lens