[This piece assumes you've seen the first four seasons of Downton Abbey. As to the fifth, it avoids specific spoilers, but does talk about themes and threads enough that you might be 20 percent less surprised by a couple of developments. It's the best balance I could strike.]
Let us get this out of the way right off: Particularly after its first two seasons, Downton Abbey has been enormously uneven. It's satisfying in some moments, dull in others, and always prone to falling so in love with a particular story beat that it cannot move past it.