I've had my PG500 for about that long. I'm not a heavy user so my build-up probably isn't as severe.
I suspect that it has creosote build-up, though. Water vapor carrying creosote condenses on the stainless steel until the SS warms up and prevents it. That deposited creosote is as hard as stone. It's probably on the inside of the chimney, too.
I think that it's inconsequential. The burn pot is as far away from the chimney as possible. I don't think there's a risk of a chimney fire (unlike a home fireplace and the need for a chimney sweep).