A good knife sharpener produces a sharp, acute edge on the knife. It should include an accurate angle guide and an abrasive surface that sufficiently sharpens and polishes.
Anyone shopping for a knife sharpener should consider the types of knives she owns. While all sharpeners handle straight edge knives, only specific models sharpen serrated knives or scissors.
Electric models tend to cost more than manual versions, but they sharpen faster and require less effort from the operator. Multiple sharpening settings provide options to sharpen dull knives or simply polish already sharpened knives. Also look for safety features to protect fingers and maintain safety during use.