Just to add my two penneth, I had one of these: https://images-na.ssl-images-amazon....L._SL1500_.jpg

Works well if you have a knife that is completely knackered, but not so good on longer knives.

Eventually got fed up with the time consuming fiddly faff of it, and got one of these: http://www.bladeplay.com/item--Lansk...-Medium--16612

It lives in the kitchen drawer, is ready in seconds and takes just a few minutes to touch up any length of knife.