i notice that throwing a nice hot pile of sparks leaves an ugly dark smudge on the contact areas of the blade. i think it is like a woodfire blackening the bottom of your pots. it is easily wiped off with a finger if you get it quick, but if you leave it for a while (like several hours or more) it can be more difficult to get off.
i find that a toothier edge (file, hacksaw teeth etc) throw a better shower of sparks if teh rod is smooth. if it is already worn ragged with grooves from using the toothier edge constantly, there is no difference.
actually, i find that if you alternate every fifteen or twenty uses between using a smooth striker and a toothed striker, it works best and throws the best sparks. i think this is because a toothy edge will bite bigger hunks out of the rod, but will eventually wear grooves in teh rod and it loses its advantage. if you then use a smooth edge, you scrape off these grooves, throwing alot of big hot sparks, until it is worn smooth. if you alternate, you maximize teh amount of sparks you get.
with all of that said, i keep my ferro rod in a small cloth pouch with a piece of glass from the bottom of a beer glass because i find that it throws really nice hot sparks, is easy to replace, and the edge is constantly being sharpened because small bits of glass flake off and expose a new edge. it is, in my experience, the best striker in many circumstances. one place that it falls short is the fact that you have to be careful with it because it will cut in all directions (watch how you hold it) and it could be considered more fragile than a piece of hacksaw blade. i counteract these two problems by using the more robust glass from the bottom of a beer bottle, and i grind one side of the piece down on a rough stone (wear safety glasses) so that if i slip it won't be tragic.
