I got a couple of these as the Outdoor Show last year from RMC and tried one for about five shots or so now. They spark very well. They seem a little softer than the agate but will hopefully last a while. It is holding up very well and you get two sides so that is a plus.
The girl up at Morris Muzzleloader sold my buddy one when he bought his first flintlock. She brought out her muzzleloader with one of those flings and sparked it a few times for him and claimed that flint had 300 plus shots on it. My buddy is still using the first side one the bought, must have 100 shots easily on it. The flint is all beat up and looks like it shouldn't work but it still sparks like a champ.