If you're considering this lens and are unsure whether it's worth it, rent one or try one on at your local camera store. Even relatively mundane scenes will show its ability. I find myself looking for reasons to use it which I've rarely done with a lens this wide. Had I bought this before the ZE 28, I probably wouldn't have bought the 28. The 28 is an excellent lens. The 21 is spectacular.
I've only barely seen the infamous mustache distortion. All other distortions typical of UWA's are very low. Used correctly, this lens can fool you into believing it's not as wide as it is.