There was a decently long line around 6 PM and we waited about 30 minutes before ordering (I suspect once the park opens up to the public, it'll be more like an hour plus). While you're waiting, there's a display of the food so you can form an idea of what you want:
The ordering set up is a bit interesting. You order and pay at a numbered cashier, then you move on to a numbered station to get your food (unless you order the Great Feast, a meal for four, in which case they'll give you a number and bring it to your table). After you get your food, you wait in another line to get seated (it wasn't terribly long so the food didn't get cold or anything but it's not ideal).
Here's everything good and bad (and British pub-like) I tried on the Harry Potter-themed menu (I apologize for the picture quality, the tavern was very dimly lit):
If you like butter, the cabbage and mashed potatoes were more like cabbaged and potatoed butter rather than the other way around. The peas were oddly undercooked and fairly hard.
The roasted potatoes were soft with a really nice seasoning to them.
The Yorkshire pudding (that biscuit-like bread, if you're wondering) was good but overly doughy and thick for Yorkshire pudding. It was best at the crust where there's a richer, slightly fried quality.
The chicken is the "safe" choice if you're looking for familiar flavors. It was deeply flavorful and juicy almost all the way through with just the right amount of tang.
The roasted potatoes were the same as you get with the Sunday Roast and just as good. The corn on the cob seems to come from the same cookbook as the mashed potatoes and cabbages and was super buttery.
I preferred regular Butterbeer over the frozen variety, which was like a butterscotch Frappuccino (or Butterbeer Frappucino). The regular version was like butterscotch cream soda with a rich, creamy butterscotch foam. It loses its fizz rather quickly, though, but that seemed essential to foam the cream top as it rises.
The pumpkin juice seemed to be a combination of apple juice, pumpkin puree, and pumpkin pie spices with a sweet tartness from the apple juice and undertones of pumpkin pie.
The only entrees on the menu I didn't try were the breakfast dishes (because they're only served in the morning), the ribs (which is more like American BBQ and offered elsewhere in the park), the Great Feast (which is a combo with ribs and chicken), Vegetable Entree (asparagus with mashed red potatoes, green peas, sauteed cabbage, caramelized onions, and roast tomato) and the turkey leg (which is also offered elsewhere in the park and at fairs everywhere).
Overall, the food at the Three Broomsticks was exceptional for theme park food. There were some hiccups here and there but the dishes there might be my favorite part of the new Wizardling World of Harry Potter area. If I had an annual pass at Universal Studios, I'd eat here over the nearby CityWalk options.
The prices were pretty reasonable as well and were actually fairly comparable to a local British pub in my area.