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!
Addendum, November 2018:
The lens have been my constant best option lens now for a long time, and while the lack of tripod mount was solved with a long Arca-Swiss style rail, another issue has crept up. Dust!
The lens is not weather-sealed, thus after time it sucks up some dust, and it is not easy to get rid of it either, although I've learned a few tricks through the years.
But it is just a minor issue, but a bit irritating.
The rail (mentioned above) screw into my camera in one end and supports the lens barrel just outside the manual focusing ring, by a small distance made out of some monosexual Velcro-like stuff. It doesn't fall to pieces as velcro does over time, and does not collect dust, another advantage. The rail is slightly oversize in length, so it can be used with the TC-1401 in place.
I use it with Nikon 1 cameras (1" sensor), DX, and FX cameras, with excellent results!
I also have the Sigma 150-600 S and the Nikon AF-P 70-300 VR DX, and there is never a doubt about which I choose unless I need a really light pack!
The iFoto tripod mount does not always fir, according to users I've been in contact with, needing a lot of abrading to fit-
Another guy, whose name I've forgotten, makes his own by 3D-printing the foot, and he sells via Amazon.de.