Because the film makers divided the book into 2 movies, they were able to include so much more of the details. The movie might have seemed a bit slow to those not familiar with the book, but for an avid HP fan like myself, I thought it was thrilling to see. It will make the conclusion of the series SO MUCH better.
The IMAX experience enhanced the visual and audio effects immensely. I'm not sure it was worth the exorbitant rate per ticket, but it was probably 50% better than watching it on a smaller screen.