View from the Empire State Building. Photos were taken during the "blue hour" shortly after sunset.
Expect looooong queues, especially if the weather is nice. You can spend hours waiting before you get up, or you can buy a more expensive express ticket and a gold-colored VIP wristband which allows you to skip the queues. The uniformed staff will assist you with that, but if there's no staff around, just keep walking and don't wait.
The first elevator will take you to the 80th floor, from which you can see all sides through the windows. If you go to the 86th floor, there's an open terrace. It can be very windy there, but you don't have to take photos through (often dirty and greasy) glass.
If you pay additional $20, you're allowed to take another elevator to the 102nd floor at the very top of the building. It's quite small and not open so you'll be behind glass again. You may want to bring something to clean the windows if you want to take photos from there, as well as something to block the light reflections in the windows (in winter, you can use your coat or jacket).
WARNING: Tripods are not allowed and you must leave them downstairs with the security (don't forget to collect it when leaving the building). I saw people with small Gorillapods so you may get away with these, otherwise bring something else to improvise if long exposures are what you're after.