I have an phase switch on my guitar. Hard to describe in words, but its very oink-ish, does not sound like a normal guitar tone at all. Its got little power and its very compressed. I would say, for cleans its great for funk and something different, for distortion, its a bloody tight crunch.
In anycase, the phase thing is bascially the coils cancelling out freqencies when wired out of phase... and the effect is greater as the coils get closer. So its really quite pointless to have a humbucker out of phase with itself. Its like almost no output and most frequencies are cancelled.
And yes, out of phase only happens with 2 pups.