I have several and find them very reliable to produce a small coal. There are plenty of tricks to make them work and plenty more in transfering the coal to prepared tinder.
I like them as interesting tools, super fun for kids and adults, but not as a primary fire making method.
Buying a good one is key....Darryl Aune makes super pistons.
I have both o-ring and thread seal versions. I prefer the thread seals. Technique and proper tinder make these very very reliable....but practice is definately required.
I have several posts on here about them...but I will let you find them. Here are some teaser shots though....