I have stayed at the Millenoum UN Plazza a couple of times. All rooms are above the 29th floor so there are fab views. The swimming pool is on the 28th floor and there is a covered tennis court on the roof.
I last stayed there a couple of years ago and got a good deal from
http://www.cityandshow.com but the advice is to shop around.
My room was on the 40th floor and was absolutly massive but the decor was a little tired looking.
The only real downsides are that the restaurent is v expensive - its geared for tax payer funded diplomats but dont let that put you off - but there are diners and other places to eat near by and that the nearest subway station (at Grand Central) is about a 5 block walk away.