This is the second best lens I have owned in almost 50 years of photography. The best is its companion 40-150 F2.8 Pro lens. A little larger and heavier then some m4/3 lenses but not so much as to be a problem for me. Optical quality is great at all focal lengths from F2.8 through F8.0.
Fast; well built; close focusing; weather resistant; sharp
EDIT: Thought of one; The auto manual clutched focus ring can be a little loose; Easy to bump it from auto focus to mechanical manual focus; Minor issue but I've done it a few times; Would be nice if it was a little stiffer to pull back