I watched BA demonstrate it in person and it does exactly what he says it does, but if the lace disappears well on your skin without it you don't necessarily need to use it.

BA gave me a bottle when I was in Windsor but I haven't cracked it open yet since I usually wear my hair comb forward, and if I do expose hairline the French lace disappears well for me. My new units are swiss in the front so they are even more undetectable.

I might try applying it to seal the lace to avoid dirt buildup since glue breaks down pretty quickly for me. If that doesn't work I'm going to try the glue dryer people were talking about awhile back.