If the knives are dull, a diamond sharpener (if we're talking about the same thing) is probably overkill but it totally depends on you. I have a miserable time getting an edge I'm satisfied with, freehand. On the other hand, I've grown accustomed to very sharp knives. A lot of people hand sharpen knives and are totally satisfied. A lot of people have dull knives too. I tried to learn it, thought I was good, and then got a kit like this: Amazon.com: Lansky Deluxe 5-Stone Sharpening System: Sports & Outdoors and realized I wasn't. Mine actually has diamond hones and cost a little more but same idea. Or pay someone to sharpen the blades once a year or so.
I would stay away from anything that has a slot or gap you drag the blade through, powered or not. They usually suck.