I have been to Vegas four or five times and stayed at different locations each time and the Cosmopolitan really has been the best so far and has raised the bar for all other hotels on the strip.
The rooms are huge. Even a standard room is plenty big enough for you and they are well kept and clean.
The soaps and amenities are nice. If you're day drunk like me you'll end up with a suitcase full of shampoos and colored pencils from the housekeeping cart.
The casino is glitz and glamor at its best. It's not cheesy at all. It gives you that Las Vegas glam but with a New York twist.
The various bars along with the waitstaff will be more than happy to get you a drink.
The spa is amazing and both gyms (one in each tower) are great. One gym is 24/7 and one is open until 8pm. Ask at checkin and they will put you in a tower of possible that's closest to the gym you want. You'll need it after knocking back all those beers and the buffets.
Don't stay anywhere else. It may be more expensive than other places but the quality is there. The service is there and really, this is a place you can spend the majority of your time at and not even leave. Stay here. You won't regret it at all!