Here is a review from a swedish very credible site, the reviewer have lots of experience from different camera systems, DSLR etc. Test is done with Pana GX8.
http://www.fotosidan.se/cldoc/fotosidan-testar/test-panasonic-leica-dg-100-400.htm
The main points are (google translate):
"Panasonic Leica DG 100-400 mm f / 4 to 6.3 is a sharp and well built, yet compact and lightweight lens that performs throughout very good. It drops only slightly in the sharpness really long distance. It's not a cheap lens, but it delivers and gives the Micro Four Thirds photographers awaited a long and sharp telephoto lens."
"The only apparent weakness is the brightness at 400 mm. It is not enough to bird photo the earliest dawn or late at night. But as telephoto package to take with you on trips or walks Panasonic 100-400 gives an unbeatable mix of range, optical quality and light weight."
"Focusing is fast and works great and silent, that which limits this is rather to autofocus in Panasonic cameras are not quite so good at following moving subjects. It will be interesting to try these lens on the upcoming Olympus E-M1 II. Thankfully, the lens has a focus limiter that reduces chasing the times when the camera loses focus. The stabilization feels very efficient."
"Puffin on Runde in Norway. The sharpness and detail even at max aperture is lovely, bokeh is good but not great. The only thing we had trouble with this evening was the focus speed to catch the flying bird, but it's the camera's fault, not the lens. (Lumix GX8 with 100-400, at 300 mm, f / 5.9, 1/640, ISO 1600)."