I would say about 1 in 3 supermarkets in the PA/NJ/DE area carry Pennsylvania Dutch Birch Beer in some variety so it's not all that rare. Most often I find it in the 2 liter bottles, sometimes you can find it in cans and once in a while you find it in glass bottles like I found it at the Dairy Queen near my home. The big difference between the glass bottled and the canned and plastic-bottled (other than not having a slight aluminum or plastic taste) is that the glass-bottled birch beer is sweetened with some real sugar while the others are corn-sweetened.
PennDutch was one of the first Birch Beers I've ever tasted, but it isn't one of the best. It definitely tastes pretty good as it hits your pallette with plenty of bite and just the right amount of sweetness, but then I get some weird aftertaste that almost ruins it for me. Not quite though. Still a good beverage despite that.
Sweetened with: "High Fructose Corn Syrup And/Or Sugar"
Verdict: Worth a bottle or two