Like most campers and outdoorsmen, I switched from matches to disposable butane lighters years ago. They're waterproof, reliable, and inexpensive. Yet these ferrocerium fire starters- the modern equivalent of the old flint and steel- seem to be more popular than ever. They're not as convenient as a disposable lighter, but they do have the advantage of being just about indestructible, and impervious to water, heat, and shock. I suspect there's also a certain romance to using a more primitive fire starting method, and more than a little association with the survivalist mindset.
Regardless, they're handy things to have, and there are a wide range of fire steels at widely varying prices available. I bought a few off eBay for under $2 each, and more recently this
Swedish Firesteel
from Amazon for around $15. There are some significant differences between the two.
The cheapest ones consist of a magnesium rod with a very small- about 1/8"- rod of ferrocerium set into the magnesium. The idea is that you scrape some magnesium off he rod for tinder and ignite it with sparks from the ferrocerium. It works, but the thin rods don't last very long. The better units consist of a solid 1/4" ferrocerium rod. That's twice the diameter, and 4x the mass.
There's another difference. The cheap units come with what looks like a piece of hacksaw blade as a striker. On mine, I had to scrape a thick coating of green paint off the metal before it was usable. The Swedish units come with a hardened steel striker that works much better. Of course, you can always use the back of a good knife as well.
My feeling is that the cheap ones aren't a particularly good deal. Assuming you're buying a fire starter as a piece of real emergency gear, you should probably buy the best one you can- especially when it's less than $20.