I consider hook sharpeners to be the most underrated fly fishing tool around. Whenever I lose a fish the first thing I check is the hook point. If it doesn't bite into my thumb nail or if I can feel a burr with my finger tip then I immediately touch it up with a swipe or two on 3 or 4 sides of the hook point. Having a sharp hook point is one of the most important and overlooked variables to ensure is on your side on the water.
This hook sharpener will fit any size fly with grooves on one side for small flies and a groove on the other for large flies. The diamond grit quickly returns your hook points to proper form.