There are pros and cons to the Empire State Building. The pros: It's one of the most famous buildings in the world, and you can feel a little thrill thinking about the fact that you're in the very place where King Kong once placed his mighty foot. Also, the view really is spectacular, especially at sunset. The Empire State Building is located on 34th Street and 5th Avenue, so you have a close but not too-close view of the other skyscrapers, a vista to the south of New York Harbor, and to the north the full extent of Central Park. If you're not really oriented in the city yet, this is a great place to figure out what's where. The Empire State Building is open from 8am to midnight, so it's sure to fit in your schedule somewhere.
The cons... may not matter, depending on your attitude. There's a lot of standing in line. You stand in line first to go through security (leave your pocket knife at home); then again to buy tickets; then to get in the fast elevator which takes you up... not to the observatory which is located on the 86th floor, but to the 84th floor, where you stand in line all over again to have your picture taken; then to reach the observatory you have to stand in line for the final elevator. The indoor portion of the observatory is all gift shop, of course, full of "I [Heart] NY" mugs and T-shirts, but there's access to the outdoors, too. During all the time you spent standing in line, there have been people sternly telling you to rent an audio tour for $6, because there's no information available at the top; this of course turns out to be false, and there are perfectly good signs explaining what the major buildings are in each direction.
The ESB is pricey. Entry for adults is $16; seniors and youth ages 12-16 are $14; children under 12 are $10 (under 6 free). There's also a "skyride," which is really a video screening on the ground floor, which you can see for an additional fee (though it's not clear why you'd want to). And then there's that picture they snapped of you on the way in, which they'll try to sell to you, for $20, as you're on your way out.
So... your call. I could happily spend a lot of time in New York without going up the Empire State Building, but that may be because I've already been up. How much are you willing to do for a great view?