The reader or viewer (this is a play after all) has a positive impression of Macbeth at first. In the opening scenes of the play he has been a loyal soldier on the battlefield for Scotland. The bloody soldier reports to the king that Macbeth has fought bravely and successfully.
Even when he hears the news for the "weird sisters" that he will be Thane of Cawdor, Thane of Glamis, and King of Scotland, we don't yet suspect the lengths Macbeth will go to to ensure his fate. The witches' pronouncement awakens Macbeth's ambition, but when an evil thought crosses his mind (presumably a desire to kill Duncan), he dismisses it.
The only negative we see in the opening scenes is Macbeth's weakness in the face of his wife's violent ambition. Although not a very attractive quality, we don't yet have a hint of the great horrors Macbeth is capable of, so the reader's opinion of Macbeth would be relatively positive in the beginning of Act I.