This is an example of why, depending on how you gauge an individual player's success, judging by playoff success is not always the way to go.
There are two sides to this argument: 1 is that a player is playing to win, and if he doesn't win, he is not succesful.
The other, more liberal verson, is that a player can not control the situation he is in (level of skill in teammates, coach), and all he can do is go out and play his best.
KG has been a great success if you believe strictly in the second version... a faliure if you believe strictly in the first. To me it is not black and white; I subscribe to a little bit of both. He is a top 5 player year in and year out and plays with his heart every game. That being said, he is far from a complete success because he has never been to the second round of the playoffs, let alone win a championship.
Referring to him being overrated; I wouldn't say he is overrated because everyone talks about what a great player he is individually, but his lack of playoff success is always mentioned in the next breath. That is why I don't think someone would have an argument saying he is "overrated." Depends on who 'rates' him though, as always.