I was lucky, I went on a Friday afternoon and there was no line. I was at the Discovery Center earlier today, however, because I wanted to see the Pompeii exhibit. The line was out the door and the staff said there was about an hour wait to get in. I don't know if that was because everyone wanted to do something indoors today because it was pouring out, or because the show is closing soon. I would buy tickets online so you can walk right in.
I should not admit this but I will anyway: I was one of the few people that were on line for the Pompeii thing, and I was miffed that they didn't have separate lines. I went downstairs where the cafe and gift shops are located, and walked into the Pompeii exhibit through the shop. Without paying. There's no staff over on the Pompeii side of the shop. That won't work for the HP exhibit, though, there's a whole slew of staff on that side.