What to wear: They say this on the website but no skirts/dresses (mirrored floors) and sensible shoes (no spiked heels or anything that can mess up the floor)
Getting there: there are 3 entrances, counter-intuitively you have to go down some stairs once you're in the building (when you're looking for a way up). The 42nd st entrance had more signage and someone directing people.
Check in: The time on your ticket is when you start the process (we got there 10 minutes early and they waved us in), it doesn't get you out of standing in line. There's a lot of being herded from one thing to another on the ground floor (ticket line, security, watch a 5 minute film, put on booties, take pictures), another line to elevator. Even though it was crowded we spent little time just standing, things were generally moving forward if at a slow pace.
Elevator: crowded but cool, your ears will pop!
Upper floors: Incredible views, we went at night and it was just amazing. The balloon room was very crowded and kind of sensory overload but my teens loved it. The bar at the top gave us some time to sit down and take a minute.
Overall: would recommend, nighttime is probably more crowded but also...those views. The other rooms are beautiful, the overall aesthetic feels very modern/futuristic. I would go again and rec it to a NYC visitor.