I was up there in november. Had to purchase tickets for a specific time in increments of 30 minutes (I purchased tickets 2 months in advance). Once you're in though, you can stay as long as you'd like. But if you leave, you'll have to buy another ticket.
I'd like to go back when they come to back around to Philly. There are some things I'd like to take more speceific pictures of. Last time was mostly props. By the time I had gotten around the models (after 250+ pics) I was so tired, I didn't really take the detailed pics I had wanted to.
The crowds aren't a problem. Most people are considerate of those of us taking pics and will move out of the way if they see you with a camera.
And don't forget to actually enjoy the display. I spent so much time taking pics, I really didn't get to see anything until I uploaded them on my computer.
-Fred