I visited the other week during the After Dark (18...

I visited the other week during the After Dark (18+) on a Thursday night. I had a blast just walking around the whole place trying each exhibit and demo. It was awesome not having any kids around, although it did still get quite busy after the first hour.
Try and give yourself as much time as you can though! I was first in line when they re-opened at 6 PM, and still hadn't seen everything when they closed at 10. It's massive!
I was travelling solo, and that was fine, but it would also be a good place to go to in a small group.