The movie itself is great, but IMAX adds so little to the experience (yes I have seen it both ways) that I must say it is absolutly not worth spending the extra money. I thought this would be true, but IMAX was the only showing available when a friend of mine could make it and I was proved correct. On the other hand, having read the books I expect IMAX will make a great difference in part two.
It has been a while since I have read the books, but I believe this one followed the book more closely than others. It is dark, but could you expect anything less given what is going on and how the previous movies have been going. A few chuckles tossed in here and there were not a distraction or out of place. I was on the edge of my seat at least half the time.