Depends on your needs
Stayed in Millennium UN Plaza Hotel for 7 nights in March 2012. The name and location is a plus, if you are to go the UN HQ. It is also possible to walk to 42nd and the most popular shopping streets if you have the strength. Otherwise taxis are cheap in NY so do not hesitate to take a cab. Walking distance to theatre district, Macy's, if you are ready for real NY walking. The hotel is not very central, the advantage is that it is not noisy there. 
The door men, concierge and bell boys are always jolly and helpful. Not the same can be said of the reception staff unfortunately. Can vary from very efficient to so-so, very friendly to "I don't care" attitude. Those who would benefit from some customer training are lucky I was so busy that I did not get time to get their names. 
Breakfast selection quite good. Staff there quite good as well.
The lobby seems too small for me considering there are so many guests, and elegant, I must say. Placing a lot of mirrors created an illusion that it is bigger.
Rooms need to be renovated. They are dated. I had a wonderful view of the UN building, the river and Chrysler, do ask for the river view when booking. I love the bed. One of the best i have tried in a 4 to 5 star hotel. The sheets they use are also very lovely. The view and the bed made me wish to go to my room. The bathtubs should be replaced. They are chipping, I just showered for that reason. The lock inside the room is very unmodern, I had to ask for technical support when I heard strange noise in my room. Apparently it is the wind blowing! And changing room will not be an assurance that the noise will disappear. It was solved by turning on the aircon/heater. It is another noise which camoflauged the somewhat "frigthening coz it is unusual" noise.
No mini-bar and tea kettle (upon request).
There is a mini-store open 24 hours nearby, where you can grab water, sandwich, etc.
Will I return to Millennium UN Plaza? Why not? It is neither bad nor ultra fantastic. As said, if you will go to the UN and want a peaceful environment, take it. But if you will go to NY for pleasure, you probably want to be nearer the attractions. Another option is Murray Hill/Lexington Ave is not bad choice for hotel location, stayed there before and it is so near to "central" and yet, still a peaceful area.