I don't know if it actually does anything, but since I've been using it, I haven't lost a new fish brought home from the LFS (the one otto doesn't count).
I only use it when I add a fish to the tank, and once in a while (probably about every 6 weeks).
My case is not a good test since I didn't start using it until after my tank finished cycling, and I assume the fish would have survived even without it.
Anyone out there know of anyone doing a controlled, scientific test?