I was lucky enough to see an advanced screening of Runaway Jury several months ago, which is coincidentally my favorite John Grisham book and is coincidentally being released today =D
Hopefully the only changes they made to the movie were from the music standpoint, as I felt that the advance that I watched was good from all aspects.
I am not saying this movie was absolutely amazing from every angle and I am not saying that it will win dozens of awards, but it was a great adaptation from the book and it had a good balance of dialogue, suspense and even a bit of action.
I would highly recommend seeing it in the theaters, as it's well worth the money imho. I will be seeing it tonight and will post my feedback and note any changes that they may have made.
The only thing you really need to know about the plot that is different in the movie, than in the book, is that the book was based around the Tobacco industry and the movie was altered to reflect the growing debate in the Gun industry.