Pentax M 28 f3.5 on K20D soft corners?
Would also be good to know what's settings your expectations. I have the M28/2.8, which is generally reputed to be not quite as good as the 3.5. When I've tested it against the kit lenses (both I and II), it easily beats the original version everywhere across the frame, at all apertures and in all respects. Compared to the version II kit lens at f/8, I'd say the corners are probably the only area where the 28 does noticeably better than the zoom, but even so, not by a huge amount. So if "quite mushy" means, "not much better than the version II kit lens", I would concur. I could easily imagine that a better lens might outperform either in the corners, but both the 28 and the 18-55 II do well enough at f/8 for me.