It is essential viewing, of course, for a first and probably every subsequent visit to New York, because then you can experience it at different times of day. We went at about midday, and the queues were much less than when we had passed by the previous day at about 3pm. Be prepared to queue whenever you go though, then queue again, and, in fact, then queue again. Pick up one of the info leaflets on the way in and you can spend your queueing time reading about how the building was constructed in just over a year and the immense numbers of people who worked on it, as well as overviews of what you can see in each direction.
However long you're standing in line, it will be more than worth it. Despite all the pictures you've seen of the views, nothing quite prepares you for what it's like. It gives you a fantastic sense of how Manhattan all fits together, and seeing Central Park in its entirety is amazing - the rest of NY may not be particularly green, although the leafy roads in East Village and a few little parks dotted around the place don't make it feel overly sparse of vegetation elsewhere, but the sheer scale of Central Park and the proportion of the island that it takes up is quite staggering.
There are audio handsets available for hire for $6, but we didn't bother with these. They will tell you that there aren't any maps or guides up at the observatory before you go up there in order to try to persuade you to hire one, but in fact there are diagrams on each side of the viewing platform with a key to all the main buildings and attractions which we found to be quite enough for our needs. Entry to the ESB is $14 for adults, and apparently the NY skyride simulated arial tour isn't recommended by those who've experienced it - it's much better to just head up the 86 floors and gaze in wonder!