I've watched him play in Serie A every year for Inter, from 2013 until now: he didn't accomplish anything and over 80% of his matches with Inter were pure garbage.
He surely has potential, but, ATM, he is not even worth half of the price Real payed for him: he has great acceleration with the ball, good dribbling, bad game vision and awful shot.
It has to be said that Inter is currently a mediocre team, and playing in Real Madrid with better players will surely improve his performances, but during his time in Milan he displayed a very, very fragile personality, which I don't think it will help him at Madrid, a team (like all the world top clubs) where, if you don't have balls, you are already half dead as a player.
He is young and there surely is potential but, at the moment, Kovacic has a big question mark on his head.