1st, I should say that I have only had the dessert at Serendipity. I've been to the NYC location countless times and have had dessert at the Vegas location several times.
My wife loves the frozen hot chocolate... I don't think that I have ever seen her order anything else.
10-year old son loves the Fried Oreo sundae, sounds disgusting but actually tastes pretty F'in good when drowned in hot fudge and marshmallow sauce... I don't think they have this sundae on the menu in NYC so this is likely a "Vegas-Special"
I always try something new and I always like it. I would guess that every sundae in this place has at least 2000 calories and probably more... I recently had a hot fudge sundae with a huge slice of carrot cake sitting on top of the sundae, it was guilt inducing but tasty. Don't come here every day while you are on vacation or your clothes might not fit!