The German folks over at Photozone tested the Olympus 12-100mm lens:
There are many ways to judge the results of the Olympus M.Zuiko 12-100mm f/4 IS PRO. If you want to phrase it in a negative way, you may call it a jack of all trades and a master of none. However, that’s unfair really. You could also argue that it doesn’t show any true weakness and that’s quite something considering the range. More so than the charts, the provided sample images may show that you can use this lens at pretty much all relevant settings with very good results at the wide end and still good results at the extreme long end. Lateral CAs are no issue. Vignetting and distortions are handled nicely (albeit somewhat lossy) via image auto-correction. And the quality of the bokeh is pretty decent for a zoom lens.
The lens gets Highly Recommended!