I just bought Noir, and the vignetting is junk. I made several pictures black and white with the app, and the vignettes (dark corners to a lighter center) are not a smooth transition of dark to light tone. The transition is made up of many discrete strips of tone that gets lighter going toward the center (especially noticeable where there is uniform tone, such as clear sky). This is what you sometimes see in low quality jpeg pictures. I hate to give a poor review, but this app puts out poor-looking pictures.
ActionRelaxin about Noir Photo, v1.01