I had a bottomless milkshake for the first time last year.
I am going back this year just for the milkshake, I want one of those really big (I mean really big) glasses that they put on the table as a joke.
I have heard that you can get it as a drink or as a dessert: but not both ways at the same time, as it is bottomless and ordering as both a dessert and a beverage for the same person would seem very strange.
They are excellent, and we, too, used them as beverages on our DDP. I believe we were told that you could order them as a beverage or as a dessert. My 1 1/2 year old drank so much that I believed she gained enough wait to put her over 20 pounds (to switch the car seat) at that one dinner.