I think Harden could be argued as a superstar.
I personally think he is a top 10 player in the league. I really don't think you can name 10 players better than him.
He is right next to Russell Westbrook in the rankings, but I am inclined to give the slight edge to Russell. The reason being is that Westbrook tends to maintain his play against the upper echelon teams and defenses in the league while Harden's play drops a brick almost. The perfect example would be the 2012 Finals. Westbrook still managed to play well while Harden struggled all series.
Harden seems to struggle when the game slows down and in the 4th when defenses are more set. This is key because in the playoffs the tempo and the speed of the game slows down and teams have to start relying on execution. I don't think I trust Harden more than Westbrook when the game slows down and if I needed a go-to bucket.
Nevertheless, Harden is a heck of a player and he is only going to get better. If he isn't a superstar now, I do think he will become one. The few areas he needs to work on is his mid-range game, his off-ball game, and his defense.
Those weaknesses are the same as Lebron's weaknesses when he was younger.

In some ways, Harden reminds me a lot of Paul Pierce. Two players that get to the line and can get to the rack at will despite not being that athletic or not looking that athletic. Both Harden and Pierce look pretty slow to me, so it is incredible how those two can get to the line as much as they do despite of that.
"Never say never, because limits, like fears, are often just an illusion." - Michael Jordan