I've had it a few times at my fav Vietnames place in NYC. The owner likes me and gives me special tastes of exotic things.
It smells really bad. IMO like a combo of garbage and gasoline. It tastes slightly less bad, nice texture, though.
Nothing I'd buy or order in a restaurant.
You can buy them fresh and frozen at the Super 88. If you have a good asian market, you've probably seen them. Big, round, with spines.
They are very pricey.