It was an enjoyable evening, but nothing earth-shattering. Anyone who was expecting Fellowes to be on his Gosford Park game will be in for a disappointment.
A big problem with the film is that there's so much to cram into two hours that things which would have flowed naturally on the show (this could have easily been spread across 3-4 episodes on the show) feel rushed, contrived, and stilted. Everybody gets to have their moment, and some get better than others (this is probably the easiest paycheck Matthew Goode has ever cashed). The story is very Mary-centric, though there's a great subplot for Thomas, Anna gets a lot to do, and Molesley steals the show at dinner.
I guess my biggest beef is that because the TV show was already pretty lavishly done, there isn't much to distinguish the movie from the TV show other than the fact that they clearly had more money to spend. We get to see a few rooms in Downton that weren't previously seen (the wine cellar looks like it might have been a jail cell 200-300 years ago), and there's some nice drone shots of the estate, but otherwise it's just an extended TV episode with more money to spend.
If this is the last time they get the band back together, then it was a nice coda to the show, but I would rather have had a seventh season of Downton than this. *** out of *****