I would have to agree. Understanding your camera and lenses, its limitations, getting the correct settings, apertures/shutter speeds, exposures, knowing how to adjust for light conditions and understanding the Histogram are the most important thing to do first before blaming the camera. Several friends have the pentax and they taken some fantastic images with it.
Ok the sigma 28-200mm lens isn't great, but if light conditions are good you should get good results. What are you trying to take images of?
Prime example, TP member was about to buy new lens because he wasn't getting sharp shots (had sent it back for servicing etc), only after long discussion, on thread form several TP members it was discovered that he didn't understand his camera settings and the camera wasn't set up right, changed that and hey presto good images....