Las Vegas is known for indulgence. Serendipity 3 should be a place to indulge. The Serendipity locations in New York and Miami are both decent places to visit. The location in Las Vegas is not worth the wait, the price, or the frustration.
If you are in Caesar's or nearby and you are craving dessert then go to the Forum Shops and check out Max Brenner's. The food a Serendipity is not that spectacular, the desserts are large but are not overwhelmingly tasty. The service is extremely spotty for a prime location on the Vegas Strip. Sometimes you get good service, other times you feel totally left out on the patio.
If you have been to the other locations, come in, buy your t-shirt then head over to Max Brenner's.