I'm using this with a Canon EF-S 55-250 f/4-5.6 STM lens mounted on a Panasonic Lumix G9. Autofocus is slower than a native m4/3 lens, but quite usable - even in low light. It does sometimes miss attaining perfect focus at long distances, in which case a second attempt does the trick. At close distance the AF nails it just about every time. Continuous AF does not work at all, as expected. Using firmware 4.0.
Overall, the Viltrox is functioning as well as I could reasonably expect, and I wouldn't hesitate to recommend it.