My local theater had three showings (one in 3D), and none of them sold out.
Actually I think I got sort of lucky, as my showing was at 6 pm and that's apparently when they tend to get slammed on Friday evenings. It was a great movie, and I have to slightly disagree with IGN's comment that the movie spent too much time in Sugar Rush. That gave the film a specific setting in which to play Ralph's misadventures, and to fill in his relationship with Vanellope. I think the movie was quite well paced actually.