They are great for a quick touch up.
Or as I said they will polish a cutter so well that you can shave in it and with it.
They are NOT a substitute for a grinder in the hands of someone who knows how to set them up really well and will never replace the good old fashioned file.
The one I have is a 12 volt.
It was handy as heck to take to the woods for a quick touch up before I learned to file left handed.
The stones that come with them are pretty sorry.
Much better ones are available from hobby shops and machine shop supply houses.
Mike