The C-Mount originated in the movie and CCTV industry and predates most photographic lens mounts. Many C-Mount lenses only cover a small image circle, resulting in heavy vignetting on Micro Four Thirds sensors — though this can be used as a creative effect. Some C-Mount cinema lenses do cover the full M4/3 frame. A fun adapter for experimental shooting.
I don't have many of these lenses and performance and artistic effects can vary.