I have a firesteel thats being shipped to my house as we speak. I haven't used it personally yet, but I hear they fall into the 'easier said than done' category(like most survival gadgets). My take on it is that they are a lifeline when you become proficient at using them and definitely worth having. But it's a little more complicated than just pointing the rod at a tree and doing a quick scrape and BOOM, fire.
I've heard they work better once they have been used a bit, so it makes sense to practice with one in the back yard a bit to 'break it in' as well as training yourself to use one. I also think the striker/scraper that comes with most are not that high in quality - and that if you cut off a little bit of a hacksaw blade to use - they create more sparks.
I haven't tried a magnesium stick yet, the one where you have to make shavings first. But they seem like they would be hell on a blade edge(not a big concern if the shit really hits the fan, but something to consider if you are just looking for a tool to help you make fires when you camp).