Cosmopolitan is a new hotel in Vegas. We are staying at Aria, so decided to take a short walk to Cosmopolitan to try out their buffet. The price is reasonable (and also offer kids price unlike Bellagio), and food was generally very good. There were a good variety of different well-made dishes, and a lot of small plates. One can tell that they paid attention into making each dish into a more complex offering, instead of the typical buffet food. There were no premium meat like crab legs, but overall the food is more comparable to a nice tapas types of place.
Service-wise, they are clearly still getting up to speed. There weren't enough wait staff in the dining room, and most of the staff seems to be busy wondering around but getting little accomplished. The manager actually initially waited on us, but when he went off attending other businesses, we were left without any service. Luckily there was a good (probably one of very few there) waiter who came by and finished off our service.
Overall I think it is definitely worth going, but either expect slower service or just wait for a few month until the system is sorted out.