The first season is great. It even made a flower contest interesting. I hope the third season is more like the first season.
Downton Abbey was a lot like Mad Men. First, you get the perspective of an interesting yet not done-to-death setting from characters on all different levels. There’s someone very high up the chain (Draper and Sterling, Lord and Lady Grantham) and there’s someone way down the chain (Peggy in the first year, Daisy). There are killer lines, often delivered deadpan (Sal, Judi Dench’s character). There are mysteries which might not add that much but what the hell, they add depth of character (“Dick Whitman”, Bates’ MYSTERIOUS PAST).
Most importantly, both settings are just on the edge of historical forces that will fuck everything up for these characters forever that we, living comfortably in the 21st century, can laugh at with the comfort of hindsight or gasp with empathy or maybe learn a little bit of history along with them. That tension is key to the success of both shows. The '60s and the cultural wars will totally remake the world Don Draper is most comfortable in. WWI is just on the horizon to wipe out a huge proportion of the younger generation of the Abbey residents. (And beyond that, Lady Grantham’s American fortune will probably be wiped out in the Great Depression, right?) A few of the brighter characters might see the changes coming, but most of them are completely oblivious.
The first season is definitely worth watching.
The second season is soap. Could be worse. Could be better.