I you have already watched all 6 season, the Christmas specials and the previous movie. You are going to watch this movie regardless of what I have to say.
I both loved and hated this movie. I loved it for all the reasons you already know, the actors we have grown to love, the beautiful scenery and sets, the costumes and the romanticism of it all. I hated it because it is the end and not in the way you are thinking. The previous movie left the world intact, we had a nice ending where Downton and the Crawely's were left standing strong. There was something left to the imagination about what the future might hold for them in the changing world. "A new Era" truly lives up to its name it breaks down the wall between upstairs and down stairs nearly completely as it also seeks to put a bow on all of the story lines. This was truly intended as the final chapter of Downton and for me its a sad way to see it go.