I sold my 12-40 after one and half year of usage. There is nothing wrong with this lens. Everything is near perfect (corner sharpness, contrast, performance at f2.8 at both end of focal length). The only thing bothered me is its weight and size. Actually this lens is much lighter than pro zoom lenses offered in other systems but I was so spoiled by portability and compactness of m43 system. So I went back to light weight prime lens and bought panny 12-32 for wide angle and zoom lens need although performance of 12-32 is no where near 12-40
Did I enjoy it during my ownership of this lens? Definitely yes.