Sorry Jef, i forgot to mention the nail file.

Yes, i've used them with great success, but it's more economical to get a pack of regular sandpaper at the hardware store and cut the large squares into small pieces. The nail files i've used aren't the metal kind.. they're the sandpaper type. We call them "emery boards" here. One side is rough, and the other side is smoother... is that what you were referring to?

Being sandpaper, the nail files work well to clean the graphite from the stump, but my preference for sharpening the stump is to use the craft knife. It's not actually a knife, it's a small triangular razor-blade which fits into a pencil-like holder. I use them to refresh the tips of my pencils during a sketch, too.