I watched all 6 seasons (52 episodes), along with both subsequent films. They've done a third film, Downton Abbey: The Grand Finale, which will be released in the UK in September of this year. They state that film will truly be the last one.
Once watching, the series quickly became absorbing after a few episodes. There were so many story lines between all members of the Crawley family and their sizable staff, that it was easy to get immersed. There were a few dullish episodes, and some not quite believable, but for the most part each next episode was anxiously anticipated, and it was sad when the series concluded.
I quickly fell in love with Lady Edith Crawley (Laura Carmichael). And it was always exciting and hilarious to enjoy any scene that the great Maggie Smith was in (Violet Crawley, the Dowager Countess of Grantham). But there was hardly a weakness in the entire cast.
Early 20th Century upper class England was a fascinating choice of subject matter. How many vast estates with numerous wait personnel there actually were during that era, I don't know, but it was fun to see everyone interacting. In real life I don't believe the British peerage would ever actually deign to fraternize in any way with their estate staffs the way some of the episodes were framed, but it was very interesting nonetheless.
The films of course can't capture the vast scope of the series, but still, I'll be looking forward to the 3rd and final movie.