Although I had a terrible experience at Aria, I cannot deny that City Center is nice. It's incredibly huge and like Dmitry said, it is very elegant and exciting at the same time. It's an architectural treat for the aesthetically inclined and it houses some of the best shops and restaurants on the strip (or future best shops and restaurants ...)
I like that they have a clear division of hotels, the entrance into city center looks like the entrance into an airport. Once inside the garage even the elevators look nicer than some people's nice houses. The carpet is plush, the colors are rich and vibrant in some areas and cool, clean and modern in others. Everything is overpriced (as to be expected) and some times the atmosphere screams "you're not good enough for this place" ... but other than that, I had a great experience exploring City Center.
My hopes for City Center is for it to grow into the hippest thing on the strip WITHOUT having the attitude.