This is the standard wide angle lens for the NC series cameras. Most people who bought the camera with a standard lens added a 28 mm wide-angle and a 135 mm telephoto lens to their system. That´s why these lenses are quite easily available on the used market at a relative low price (you can get a mint one for approx. US$ 50).
The performance of this lens is quite good, pretty standard for a 28 mm lens from a reputable manufacturer such as Mamiya. Nothing really exciting but very useable. Corner sharpness could be a little bit better even stopped down. Sharpness in the center and contrast are very good.
Overall, it doesn´t perform quite as good as the 21 mm lens which is very surprising.