I'm new to all things pi and just got a Kuman 7" 800 480 LCD screen. I tested it on my laptop first (windows 8.1) and it looked great. Plugging it into the pi, not so great. The colours are all off, only part of the screen is used and image quality is awful. I can see enough to use it but would prefer it to work properly.
I've followed the instructions from Kumantech's site here reinstalling raspbian (not sure which version) after making the changes to the config.txt. Nothing has changed though. The screen image from the pi is still terrible. The touch screen works (sort of) but needs calibrating.