Never having been to NY, the only way we knew about the original Serendipity's was the movie of the same name starring John Cusak a few years back. So finding #3 in Las Vegas we had to give it a try.
We both went for their original claim it fame, the Frozen Hot Chocolate. Quite good, but not quite what I expected. It's served in a sundae-type bowl topped with whipped cream, but is actually of milk shake-like consistency - I drank mine through a straw. Tasty, but I would have preferred it to be served in a traditional shake glass.
They have many other flavors of the frozen hot chocolate and a large menu of food. Strip prices: burgers are $16. Long lines are typical, but you can order at the outside window if you don't need a table (and since the inside decor is strictly ice cream parlor standard, you wouldn't be missing much.