The opening sentence in Macbeth's letter in Act I scene 5 of Macbeth makes it clear how he feels about the Weird Sisters. What is his opinion of them?

This is a very interesting question. In the letter that
Lady Macbeth receives from her husband in this scene, Macbeth clearly points towards the
way he believes in the power of the witches and their prophecies. It is clear that
Macbeth has decided at this stage to completely believe the witches and to not worry
about if their prophecies are evil. The truth of their first prophecy, which declared
him Thane of Cawdor before he was actually awarded that title, has convinced him of
their veracity. Note what he says in his letter:

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They met me in the day of success; and I have
learn't by the perfect'st report, they have more in them than mortal knowledge. When I
burn'd in desire to question them further, they made themsevles air, into which they
vanish'd.

What is important
to note is that at no point does Macbeth reveal his suspicions that the witches are evil
or malign spirits. His ambition and desire for power seem to have overpowered or blinded
him to these other questions or scruples. He only focuses in his letter on how they
"have more in them than mortal knowledge." This seems to indicate that even if Macbeth
had proof that they did have an evil intent, this would not change or effect the impact
of the prophecies upon him.