Summary: I bought my KX used in 1977. It was my second 35mm SLR (after using an old Praktica for a few years), and I have literally taken tens of thousands of photos with it. I was only 14 when I bought it, and it was really a top-of-the-line 35mm at that time. As a school newspaper and yearbook photographer, I got a ton of experience using the KX and it was an ideal camera for a student photographer.
Pros: Absolute reliability. Built like a tank. Superb metering, huge selection of compatible lenses, including the wonderful Pentax SMC line. Also, any Prakica/Pentax screw mount lens is easily adaptable to this camera. The only, minor, fault that I can think of for this camera is that it's exposure meter uses up batteries relatively quickly, but as the batteries are cheap and readily available, this is of minor consequence. Best of all, KX's are common today and inexpensive.
Cons: The aforementioned tendency to use up batteries quickly--hardly a liability. I also would have liked a split image focusing screen, but in practical terms, I never found the existing focusing aids to present any sort of liability. A great, great, camera.