The Panasonic GX8 is particularly good for using with manual focus lenses. Add in the Metabones 0.64x Speed Booster and there is the possibility to use some high quality lenses with the equivalent of some fast apertures, thanks to the extra stop of light the adapter gives. This also brings to m4/3 the possibility of a 'full-frame' look. Now I'm not especially a fan of the narrow depth of field look, but there are certainly many occasions where it can prove very useful. These combinations give an approximate 1.3x crop, the same as the Canon EOS 1D and Leica M8. So not quite 'full-frame' but close and obviously closer to that look than from a 1.5x APS-C sensor.
At the top of the page I've put some images of the Nikon prime lenses that I use. I've had the Voigtlander 58mm f/1.4 and Nikon Series E 100mm f/2.8 for a while and have added a Samyang 14mm f/2.8 and 85mm f/1.4 in recent days. Samyang (and their other brand names) make good value MF lenses that are 'old school' in terms of design and construction. They are even now offering some 3 and 4 lens kits specifically for use in shooting video, with T stops and variable, stepless aperture control. Now for the kind of video I shoot, this is not something I need, but for those who do, it's a very cost effective way of putting a decent set of primes together.
Above is an example of how these Samyang lenses work for video. Below I've got some examples of how they work for stills.
Above is the 85mm wide open and while it's not biting sharp, it's a decent performance.
The image on the left above shows the performance wide open again and the bottom sample has had some sharpening applied in Photoshop, which works well. However, as you can see from the comparison on the right, stopping the lens down produces an excellent result.
The 14mm example above shows again the value of stopping the lens down in terms of sharpness, but again wide open it's very acceptable.
Finally just to give an idea how these lenses perform as 'full-frame' lenses on a 'full-frame' sensor, here are some comparison samples with the lenses on my Nikon Df.
So as you can see the 0.64x speed booster gets more in the image than a 'native' m4/3 lens of the same focal length and also using a passive adapter, but the Nikon still uses the whole of the lens area.