I've used a Stanley 10-499 quick-change retractable utility
knife forever, for all the usual tasks - sharpening carpenter
pencils, opening boxes, scoring drywall, and cutting building
paper, nylon line, composition shingles, and insulation. It
slips easily into the holder in my tool belt, and spare blades
stack in a plastic tray that swings out of the handle at the
press of a tab. Blades are installed and removed by shifting
the slider all the way forward and then pressing a button on
the nose of the knife while slipping the blade in or out.
I've stuck with this...