You can directly use any Pentax K-mount lens on any Pentax digital body without the need for an adapter. You can also use any screw mount Pentax lens on a digital body with a small cheap adapter. You can use K-mount or Screw mount lenses on Canon bodies with adapters, but not Nikon. If your K-mount lens has an A setting, any digital Pentax will be able to automatically control the aperture, but of course any manual focus lens will still be manual focus, and any lens without the A aperture setting will need manual aperture control.
Whether it's worth the effort depends on which 50mm you mean. What kind of lens is it? You can look up more information on your lens here:
Pentax Lens Review and Specification Database - Main Index