Lenstip (Click here) tested the lens and writes:
The Panasonic G X VARIO 12–35 mm f/2.8 ASPH. P.O.I.S set the standards pretty high in this category but the Olympus company managed to meet them without any problem. First of all they produced a bit cheaper lens with a wider focal range. When it comes to the image sharpness the Panasonic is a bit more even but the Olympus provide a tad more lpmm at the maximum relative aperture. Apart from that you get slightly lower distortion and chromatic aberration, noticeably lower vignetting and distinctly better performance against bright light. The Panasonic, for a change, corrects the coma better and its off-focus images look nicer; it is also physically lighter and smaller, despite built-in image stabilization, and it is produced in Japan.
The sharpness is overall pretty good and homogeneous (even at f / 2.8) especially at wider focal lengths. Strangely 40mm sharpness at full aperture drops significantly vis-à-vis other focal lengths. Everything returns to normal as soon f / 4.