Really excellent show. I began watching after Peter Capaldi was cast as the 12th Doctor and I am very glad I did. Series 1-3 are more light-hearted whilst series 4 is more serious.
The ending is... sad. But not disappointing. The last scene of a certain character driving away in a taxi is moving and you'll be thinking about it for a long time afterwards.
Favourite episode: Series 3 finale. Malcolm is wonderfully evil.