What makes it so great? It's possible to taste individual flavors like Vanilla, Liquorice and Mint in the beverage -- something the mass produced brands like A&W did away with long ago in favor of artifical flavorings.
We bought a bottle, but haven't tried it yet.
My ten-year-old daughter and I really enjoyed Butterbeer, both the frozen and the "cold" versions. Her preference was "Icee-style" as we called it, while I believe I preferred the mug full of creamy, cold-but-not-frozen goodness. Yes, we sprang for the mug.
I think we both agreed that the Flying Cauldron version was kind of pegging the sweetness meter, at least for our taste buds. The cream soda element was much stronger than I remember from the Park, and also, I thought that the original butterbeer had just a touch of a root beer twist to it which was not present at all in the knock-off.
Also, here in Hawaii, they'll hit you up for $3 per bottle! No real surprise there, if you've ever been here or lived here. Everything is more expensive.
All in all, we enjoyed it, and it brought back awesome memories of our day at Universal's Islands of Adventure, but I wouldn't rush back over there to grab another bottle. I suppose if we ever had any reason to have a Harry Potter-themed party, though...