Richard Wong (Click here) published the full Leica 100-400mm lens review:
The Panasonic 100-400 really highlights one of the strength of the micro four thirds system. A lens with insanely long effective focal length but with reasonably compact size. The build quality and image quality of the lens are decent and I really like the dual lens hood and lockable focal length design. Not to mention that the really fast autofocus speed and good O.I.S. /dual I.S. performance. I do wish the integrated lens hood is a little bit longer so I don’t need to touch the detachable lens hood most of the time. But the only thing I don’t really like is the tripod collar can only be rotated by 90 degree so you do have to remove it when shooting handheld. The smallish f/4-6.3 aperture size also limits the low light performance (both autofocus speed and image quality).