I don't know how good they are but i figure that if The Edge, David Gilmore, and Andy Summers all use one to get their tone before the signal hits the amp, then it's gotta be good. If it wasn't as good as ppl say than why would those three guitarists use them for over twenty years. I personaly have never seen one in public only on ebay.
I found one in a shop selling old stuff.I got it for $US20 and was pretty cool piece of gear.The signal was pretty clear and no coloration.It was designed to be buckled somewhere behind you so that you can adjust on the fly.I can understand why it had that aura or mojo,history of players using it but honestly it bored me to tears.In the end i swapped it with a sparkling green zvex fuzz factoryNever been happier.
I'd like to get my hands on a faithful clone of one too, but I've got to calm down a bit. I've got so many pedals now that I think I'm going to have to rent a room in someone's house to keep them in!
I have an FA-1 that I bought somewhere in the early 80's (I think). I originally used it to boost the treble side of a Chapman stick but lately used it for boosting an acoustic guitar. A very nice clean boost - but I never thought of using it for my electric setup.