Love him or hate him, he's almost as much Doctor Who is as the Daleks. And he even got his own television program. Twice! He should have turned up in Torchwood just to make the entire collection
There's this great episode of The Sarah Jane Adventures where the kids are checking up on SJ, and they entrust K9 to keep their secret. Of course, he bails and just says outright, "Mistress Rani has instructed me not to tell you that you were followed this afternoon."
That was "The Wedding Of Sarah Jane Smith" which was a great story (Although I don't think SJA did a bad story to be honest). Interesting how they got to use K9 for that series, though
No there are some fans out there who hate him. Kim Newman? In his introduction to the last "Pocket Guide", he said that classic Doctor Who was crap from the moment then introduced K9, and suggested that the casting of Matt Smith, was the K9 moment for new Who. So sadly, K9 not tremendously appreciated by some.