Have had half a dozen knife sharpeners in the past and they all seem to scratch the sides of the blade.
I recently forked out (sorry about that) good money for an I O Shen knife, so with that in mind I found myself looking for a really good knife sharpener recommended by said manufacturer.
When this arrived I thought I'd practice on my cheaper knives first.... WOW!!
Ok they may not hold there sharpness as long as the I O Shen, but WARNING your ordinary knives will have you investing in large amounts of plasters if you're not very careful.
Very little practice will renew all the knives you have, for a very long time.
And the good news is, after a few years you can replace the internal grinders for just a few pounds.
A great investment.