In Act III, Scene I, Macbeth reacts to his success, with fear and suspicion and plans for yet another murder. Although he hosts a banquet, his highest point as King, even that is not happy. Instead of being able to enjoy the company of his friends and family, he is disturbed by Banquo's ghost.
In my view, Macbeth never enjoys being King. He never has a chance to, his mind is too consumed with keeping his crown to really enjoy it. As a result of his preoccupation with controlling the future, Macbeth continually engages in activities, such as, killing Banquo and consulting the witches for a second time, that cause him to ignore the present.
Macbeth is never truly comfortable with his success, he is haunted by guilt from the beginning, since he murdered to gain the crown. His personality, that was formerly defined by characteristics such as courage, loyalty, and true hero of Scotland, is lost in his desire for power and his manipulation to get it.
Macbeth loses everything that makes human life enjoyable through his actions. He is, therefore, unable to enjoy his success. The simple joys of life are denied him, once he begins his murdering spree.
He is truly miserable with his "success."
Macbeth responds to becoming king with concerns about his ability to hold on to his new position and authority. As has been noted, Macbeth doesn't relax once he becomes king. In Act 3, Scene 1 he worries about the Weird Sisters' prediction for Banquo: that he will not be king but he will father kings. Therefore, about becoming king, Macbeth says, "To be thus is nothing, / But to be safely thus. Our fears in Banquo / Stick deep [...]" (3.1.52-54). Macbeth feels that it is meaningless to be king unless he can feel safe in his role, but because of what the Weird Sisters told Banquo—and because Banquo is a man of such strong character—Macbeth worries that his power is not secure.
Further, Macbeth is angry that he has burdened his own conscience and soul by murdering Duncan, just so Banquo's descendants could sit on his throne. He says, "For them the gracious Duncan have I murdered, / Put rancors in the vessel of my peace / Only for them [...] / To make them kings, the seeds of Banquo kings" (3.1.71-75). Therefore, Macbeth really cannot be contented as king: he's too busy worrying about the security of his crown and feeling bitter about his former best friend.