We were lucky enough to visit just after the renovations in 2019 were completed and couldn't be happier with the experience. (better than Top of Rock by far, but you go there to take a photo of the Empire State!)
There are loads of reviews for the old building and I really can't add anything new except to say the experience, the staff, the new refurb were all excellent and classy. Old world charm from the staff and building.
Don't forget to get a photo with King Kong!
We were there early October about 11am and nobody else was. Bought tickets online and walked straight up, no waiting. Wandered around the exhibit and interactions before going outside, blessed with a glorious day. There is a downloadable App for those that like to walk around and listen to a self guided tour, like my wife, which is a great bonus.
Very pleasant experience all round, and a big thanks to the staff who went out of their way to be jovial and interact with us. Maybe because no one else was there and they were having a comparably easy day for them, but we'll take it!