I've sharpened mine a few times using a regular bench grinder. As they come from the factory, they seem to have a fairly dull cutting edge on them. With a grinder, you can make the edge quite a bit sharper, which makes them work a heck of a lot better.
These things work pretty well until they hit a nail or other metal object. It is a good thing that they resharpen easily, because it is easy to wipe them out if you hit metal with them.
