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....


