I just finished watching the first season of Downton Abbey. I really enjoyed it, almost assuredly in part due to the historical setting, the same reason I love Boardwalk Empire, Mad Men, and a few years ago, The Tudors. The sets and costumes are all fantastic, and the characters are very well done. They really draw you in to care about them, making up for the lack of intrigue or high stakes which is so key to the appeal of shows like Boardwalk Empire, Lost, or Dexter.
That said, there are a couple of glaring issues for me. First of all the nobles are way, way too nice. They all seem very concerned with the well-being of all of their service staff, which seems completely inappropriate for that time, especially when the concern seems so universal in nature. Even the more self-interested of them take care to be kind to even the most lowly of servants.
The show also seems to be taking liberties with time. The first season takes place over more than two years, from the sinking of the Titanic to the beginning of the First World War. However, nobody seems to have aged much (beyond the cook), and sometimes characters pick up conversations from one episode to the next, even though months must have passed in the timeline.
Still, it's definitely worth a watch, particularly if you like British accents and historical settings.