A sharp blade is critical for the performance of your knife and your safety while using the knife. This applies to any type of knife - chef's knife, paring knife, pocket knife, survival knife, switchblade, or even a machete. A sharp knife is important for safety since a dull knife blade requires more force to do the job than a sharp knife blade. The bottom line is that if you have a knife or knives, then you absolutely need a knife sharpener to keep your knife blades in top form.
Here is our table of the best knife sharpeners under $100. The knife sharpeners on this list offer basic functionality for the average knife owner. I would be happy to have any of these sharpeners as a first step into knife sharpening. If you are looking for a top notch knife sharpener for your kitchen, I would highly recommend any of the electric models listed. The speed in which you can sharpen a knife, a dull knife, is typically about 90 seconds to 2 minutes. If you just need to touch up a blade before a furious session of onion chopping for a big pot of chili, then any of these electric sharpeners will take under 30 seconds.
If you're a DIYer and you know you do like sharpening knives already AND you have plenty of time, take a look at the Shun Whetstone. It is highly versatile and provides unmatched flexibility for the angle of your blade. This is how your ancestors sharpened their sharp implements and it doesn't need to be plugged in. It does, however, take lots of practice and patience. You might end up in a zen like state. That's a good thing as long as your expectations are set properly. I would definitely have some practice knives before you go and try to sharpen your Shun Ken Onion Chef's Knife.