I spoke with Michael Brooks a L-O-N-G time ago. He was deliberately unclear about the Infinite Sustain thing, because 1) there was some issue of patent/exclusivity not yet resolved and 2) it's a friggin' dangerous rig, apparently runnung 120 volts right into your guitar. Kinda like running around with a toaster on your belly. I believe in terms of tone and response, the Sustainiac will do the same thing without making you feel like you're waiting on a phone call from the governor to play the second set.
My experience with both the Fathead and some homemade variations was this: it adds a TASTE more sustain, but barely tames the sometimes wild resonant frequencies that some (usually bigger) headstocks can gobble up. I remember working on a Fender 5-string bass with n oversized head that made the high D (G string, 7th fret) disappear like a Houdini trick. The only amount of mass/stiffness that would counteract it was a C-clamp - about five pounds! If you had an axe that you simply couldn't live without that had this kind of problem, you might consider some graphite rods in the headstock for stiffness sans weight.
BTW, Aspen and Associates, makers of the fathead, is Aspen Pittman of Groove Tubes fame. A good man with golden ears who will talk YOUR ears right off your head.
Douglas Baldwin, coyote-at-large
"The music business is a cruel and shallow money trench, a long plastic hallway where pimps and thieves run free and good men die like dogs. There's also a negative side."
--- Hunter S. Thompson