I have to agree – my 12-40 Pro has come in extremely handy during my photography excursions in Norway, our local national parks, and the Dolomites. The image quality I'm getting from it is fantastic, and it also eliminates the need to switch lenses. This especially is crucial for me: often, the environment doesn't really lend itself to lens switching (wetness, rain, dust, wind, cold numb fingers...) and the 12-40 can handle almost everything that I need.
This year, I bought a used 8-18 PanaLeica to get those wide angle shots with a lens that takes filters, but I found that switching between this lens and the 12-40 became too cumbersome – it's exactly what I didn't want in the first place, and there's also too much overlap between the two lenses. So I'm selling the PanaLeica again, going back to my Samyang 7.5 Fisheye on a second small body, and will stick with my faithful 12-40.