I've built/used them in the past, and still have the parts in my shop. I consider it one those "use in a pinch" things.... you'll burn through the torch fuel bottles quickly. With a standard propane torch as the burner, it only works on very small pieces of steel, and then sometimes not very effectively.
Make sure you use SOFT firebrick! I've had a couple of folks contact me in the past, fussing about how much trouble they were having trying to build one....and only after several minutes of conversation, did I discover they were trying to use "standard" (hard) firebrick(s).
I tried to use a single brick the first time I did it, and ended up with a pile of crumbled firebrick..... then I put two bricks together, held with iron tie wire, and used a wood boring bit on a drill to bore a 2" chamber..... then a 1/2" wood bit for the burner hole.... worked great.
Because it burns through the torch fuel bottles so quickly, and the fuel pressure in those bottles is so low, I built an actual venturi burner from 1/2 black iron pipe, and hooked it to a 20lb propane cylinder (if you do that, make sure you have an adjustable regulator on the tank)..... that made a huge difference. After doing that, it was basically a true "mini forge".... I could even weld in it.
It's fun for tinkering around, and you can work on very small things, but it's no substitute for a full sized forge.