I heard the Cosmopolitan was great, but I was still surprised and impressed. Simply a fantastic hotel with a great location and even better service.
I had a 55th floor room overlooking the Bellagio fountain. Great view overall! There are also rooms with a balcony (single king) and suites. The bathrooms are chic, the decor is modern, the amenities are upscale, and every bit of service I received was simply top notch.
The casino here may not be as big as some of the other Vegas big names, but you can find everything from slots to blackjack to war, so there is plenty of diversity. The restaurant selection is great, and both pools are wonderful. I didn't get a chance to check out Marquee, but it seems like everyone has a great time there.
If you want to be/feel hip and stay in a really nice hotel (but aren't throwing gold bars around just yet) then the Cosmopolitan is definitely the place to be!