Located at the new Cosmopolitan Hotel, I went to both the Marquee Dayclub and Nightclub -- and they were both awesome.
Dayclub:
I love pool parties and I've been to the different ones like Encore Beach Club, Tao Beach, Rehab, and Wet Republic, but Marquee Dayclub definitely tops them all. I love, love, LOVE house/trance music and that's all they played. Drinks were delicious, especially the frozen drinks. (I highly recommend the "painkiller") The cocktail servers were attentive. The pool was smaller than some of the other pool parties, but they had beach balls and everybody was playing water volleyball with each other.
Nightclub:
It's definitely A LOT bigger than it looks. The interior design is classy and posh. Drinks were pricey, but it IS Vegas... the music was good on every different floor. They even had a "library" where there were chairs and pool tables. I prefer XS over Marquee for nightclubs, but Marquee is definitely up to par.
I would definitely recommend stopping by Marquee if anybody plans to go to any pool parties or nightclubs in Vegas!