I have the mid-size and mini fire steels (but have used the larger size too). The big thing about the little guy is that you obviously have fewer strikes. My guess would be 1000, depending on how aggressive and hard you use the striker. The mini came about because one of the editors from Backpacking Light asked the manufacturer to downsize it for ultra light backbacking. The smaller one means you have to be a little more delicate and deliberate with the firesteel than with the larger models.

I keep mine on my keychain and put the striker in my archive storage. It isn't that the striker doesn't work as well because it does - I just elected to not carry it on my keychain with the rest of my stuff. I can use either a carbon steel knife or the edge of my lock on the lockback knife to create the sparks. I think using the edge from a hacksaw blade would use up the ferrocium quicker because of the teeth but I don't know if you would get better sparks.

I've used it on my camping stove and a couple times on tinder with good results each time. I used a bit of plastic tubing from my aquarium supplies to wrap the ferrocium rod but found out that it really isn't necessary and it makes it slightly less friendly to use.

I've heard of people who have had difficulty using these strikers but it always comes down to the fact that they aren't using it properly. A strong swipe of the length of the rod instead of a quick hitting strike will always yield better and more sparks.