There are all sorts of knife sharpeners out there. Some are electric and some are mechanical but all serve the same purpose, they sharpen knifes. Mechanical knife sharpeners can be difficult to operate and to get a smooth sharpen but with a steady hand and some practice it is a great option. If you are looking for a reliable sharpening tool, consider purchasing a Smith's Diamond Sharpening Stone or a Smith's Diamond Sharpening rod. These are both great choices for a knife needing routine sharpening. Keeping you knife sharpened over time will prolong its life. Also of note, a sharp knife is a safer tool to use than a dull knife because the user can allow the knife to do the work rather than pushing or forcing the knife which can in turn cause the blade to slip. Many cut fingers and other line-of-fire kitchen accidents can be attributed to a dull blade.
There are more complex units on the market with motorized electric sharpeners which are handy for sharpening a number of different types of knifes. The electric sharpener is a great choice for those who are not comfortable with operating the manual sharpening tools, as there is certainly a safety component involved. There are a number of different models serving the same function and purpose all for an approximate range of $70 to $200 depending on the model.