It makes a dull knife sharper and it does pass the potato test.  It doesn't fare so well against tomatoes and copy paper.  What is more concerning is the reviews of the replacement blades, which point out how quickly they wear out.  I use this on my cheap steak knives that I don't mind subjecting to the abuse of the dishwasher.

This thing loves to grind metal off the blade like crazy. I originally suspected it of leaving chips and pits, though I can't actually confirm it does.  From what I can tell the damage was already there.  That said, keep this far away from bread knives with long teeth as it tends to knock down and bend the serrated points.  This is where I'd consider using an electric grinder.

Take heed from the 1-star reviews, don't buy this for your good knives.  It will wreck the edge on the blade and simply cannot get it sharp enough for most prep work or heavy duty use.
It makes a dull knife sharper and it does pass the potato test. It doesn't fare so well against tomatoes and copy paper. What is more concerning is the reviews of the replacement blades, which point out how quickly they wear out. I use this on my cheap steak knives that I don't mind subjecting to the abuse of the dishwasher. This thing loves to grind metal off the blade like crazy. I originally suspected it of leaving chips and pits, though I can't actually confirm it does. From what I can tell the damage was already there. That said, keep this far away from bread knives with long teeth as it tends to knock down and bend the serrated points. This is where I'd consider using an electric grinder. Take heed from the 1-star reviews, don't buy this for your good knives. It will wreck the edge on the blade and simply cannot get it sharp enough for most prep work or heavy duty use.