With 2,818 reviews and climbing, the Lansky PS-MED01 BladeMedic is one of the best-reviewed things on Amazon.
And you can add one more to that number: I have one and love it. Fairly lightweight but high quality, this is, hands down, the most useful effective portable knife sharpener.
Lansky knows how to make a high-value deal. The BladeMedic sharpens any steel blade, including knives with fine serrations, beautifully with zero risk of ruining your blades. Check it out:
Seriously guys, your only excuse for not having one is either that you didn't know about it, or it was out of stock when you last checked. For many people, this can easily serve as the only knife sharpener you ever need.
Whether you're using it in the field or in the kitchen, this is an indispensable tool that makes a thoughtful gift to boot.