Marquee is probably the hottest nightclub in Vegas right now and it is due to a few things - it's the newest club on the strip located at the Cosmopolitan Hotel and so with being new, everyone wants to go and check it out. It is also being run by the Tao Group (who also own Tao and Lavo - in both Vegas and NY) and they know how to run clubs.
Overall, I think Marquee is a nice club, but sometimes does not live up to the hype. There are 4 different areas you can party/dance/hangout/drink. The main dance floor area is where all of the action is but is overly cramped - way too crowded for me to enjoy myself and so I find myself exploring and hanging out in other areas of the club. There is the Boom Box room where they usually play hip hop and the Library room which is more chill and not so club like. The outside area (which is Marquee Day Club during the day) is usually where I end up (mostly because that is where my table/cabana has been lately). The vibe is always going on at Marquee and it is always crowded - be sure to go early if you are not getting a table/bottle service and be prepared to wait. You can get pre-sale tickets online through their website and that is a very good idea especially on long weekends or nights with big name DJs.
The pool parties are also very good and a lot of fun and sometimes I enjoy these more during the summer than actually clubbing. Nevertheless, another great addition to the nightclub scene in Vegas.