I purchased a sharpener when they first came out years ago - and thought it was a total waste. UNTIL a friend told me that it worked best to KEEP it sharp. Now, I sharpen blades really, really often from the time I take them out of the package, and I get great mileage out of my blades. I've replaced the emery/sandpaper wheel a number of times over the years for far fewer $$ than the savings on blades.
One caveat - the sharpener will not take out nicks. If you've nicked your blade, you may as well toss it.