Interesting. The last one I had, my buddy brewed as a pilot batch for a small brewpub he was working for. The finished product lost its head very quickly, assumingly from the PB2.
PB2 is defatted peanut flour, essentially what is left after extracting peanut oil, but I wouldn't be surprised if there's still enough oil left in it to effect head retention.
As a food scientist, I hate this stuff (in beer or other wise) because they claim you just mix it without water, but without the oil it's just NOT peanut butter.. in a legal labeling sense or an organoleptic sense, but that's just me. Some people swear by it as a low fat PB alternative. It's also ungodly expensive for what is essentially a waste product.
I've had a couple of peanut butter beers and they are interesting, but gimmicky. Not the kind of thing you could drink several pints of (IMHO). Belching Beaver makes a chocolate peanut butter beer that was pretty interesting, but it was a 3 oz sample at a stout festival, so again just the novelty of taking two sips like "Oh, that's pretty interesting" don't think I could or would drink a lot of it.