I bought the Panasonic GX8 a few days ago, along with the 12-35mm f/2.8 lens. After the GH3, Olympus E-M1, and GH4, it seems that the GX8 has a couple of interesting features, including the tilting EVF and the Dual I.S.
Previously, the 35-100mm f/2.8 I dislike so much was the only lens I had with Optical Image Stabilization (OIS), but I changed that with my purchase of the 12-35mm f/2.8 lens. With an extra US$200 off, it seemed a useful purchase, rather than buying another Olympus 12-40mm f/2.8 lens.
When viewing video I've taken with the 35-100mm lens, I'm often reaching for dramamine because the OIS doesn't seem to work well, especially compared to lenses that don't have OIS. It is a compromised design that probably should have been much, much bigger. Instead of the 58mm filter size, it would probably been sensible to make the lens big enough for a 67mm filter size. The 12-35mm lens is equally small but feels a better build.
Around 8:30 p.m., earlier than the usual 10 or 11 p.m., I took the GX8 out with the 12-35mm lens mounted. I wanted to see how the Dual I.S. would work.
I would say that I'm not terrifically satisfied. Hopefully, firmware will improve the functionality. At this point, I switched to the Panasonic/Leica 15mm f/1.7 lens.
Hopefully, the in-body Image Stabilization is working well. I'm not entirely convinced but it probably works as well as that of my Olympus E-5 from 2010.
Update 2016.11.13: As I've seen with the newer Panasonic bodies, the IBIS from the smaller GX85/80/GX7 Mk II and G85/80 is much better, plus they've implemented an electromagnetic-driven shutter mechanism to correct for shutter shock or whatever the term is.
Out of all the comments I've seen, only one person has said that he gets some value from the GX8's IBIS and he said that he got better shots than with the GH4. That is not my experience, unfortunately, nor does it seem to be the experience of the majority of commenters. I have better shots with the GH4, and certainly with the Olympus E-M1.
If it works, I can't tell any difference.