Much like a good pair of scissors, having a good utility knife will give you an edge in trimming materials for your projects. Utility knives cut where scissors can't. For projects that involve trimming around tight corners, a utility knife can keep your edges smooth. Having a quality knife in your supply drawer will save you time -- and sanity -- on a variety of projects.
Of course, if you have a nice set of utility knives, you're also going to need a decent rotary cutter. A rotary cutter is similar to a utility knife, but it has a round blade that rotates as you pull it through the materials you want to cut. This allows you to cut paper, cardboard, mat board, cloth or fiberglass smoothly.
Plan on spending around $15 to $20 for a rotary cutter and each utility knife. We know, $15 or $20 likely won't break the bank, but depending on your hobby (and craftiness level) you may need multiple knives, and that can add up really quickly.