The fairly new Sigma 100-400mm F5-6.3 DG OS HSM (introduced in February 2017) is a pure delight, far better than the Nikon 80-400G I used to own, and costing less than a third.
Compared to the also impressive Panasonic-Leica 100-400 it is marginally bigger but lacks a tripod mount (the only drawback with this Sigma lens).
What really differs is the smoothness of the zooming, which is silky-smooth with the Sigma, and needs an effort when you use the PL lens (it get's better over time, but never gets at all smooth).
As yet I have tried it on my Nikon 1 and DX cameras, with no issues at all. A bit slower focusing when used with my Nikon 1 cameras, but spot on, so who's complaining?! I see no issues using it with my FX bodies, but that will have to wait.
It even works with the Nikon 1 J5, a rare occurrence, as most long zooms are incompatible (and quite a lot of other lenses as well are also incompatible).
Compared to the lovely Sigma 150-600 S, that I also own, it weighs about a third, is much smaller, has the same switch layout, is also compatible with the USB dock, and seems to share the same manufacturing standard as the 150-600 Sports.
In short, small, and impressive!