Bill,

   I've been using them for more than 20 years, myself.  I needed smooth
strong threads in a rear sight I was making for a mini-14 rifle.  Since then
I've been using them on nearly all aluminum parts I make for aviation use.
As soon as my new "tap" comes in, I'll be doing them even for the NPT
threads in my oil filter adapters.

You should check in Machinerys Handbook or with the manufacturer for the tap drill size. I always ask for a tap with a air/lube relief groove. Hand tapping can be tough with the roll tap as these are called.

Bill Jepson 

The roll or form tap works well on most metals. You can form tap steel and cast iron too. The technique shows the most improvement over cut threads in soft metals like aluminum, but it makes a stronger thread in almost anything. Check the minor diameter of the thread after form tapping. It should be within normal specs. The other thing form taps are good at are blind holes. Since there are no chips there are no chips left after tapping. The cleanout time alone is worth using form taps at some machine shops! Be sure to put a stop around the shank though because if you bottom the form tap you will break it for sure. Since the tap is so well stuck you will NEVER get it out without EDM so be careful!

Bill Jepson