The Hollywood Reporter is reporting that the premiere drew 5.3 million total viewers and 3.6 million viewers in the 18-49 demo, the only demo that the advertisers, and thus the networks, really care about. Not only is that the biggest 18-49 audience for any series premiere on any cable network this year but it's also the biggest numbers for any series in AMC's history.
UPDATE - When you add in the viewers from the replays at 11:30pm and 1am, the total viewership rises to 8.1 million.
By contrast, here are the total viewerships of the first episodes of some other AMC series:
Mad Men – 900,000
Breaking Bad – 1,343,000
Rubicon – 1,700,000
Talk about the show itself here. Talk about the ratings below.