What does Banquo think about the witches' prophecies?

At first, Banquo is a bit in awe of the Weird women.  He remarks on their appearance and actions, wondering if they are truly earthly or supernatural in nature.  After the sisters tell Macbeth what his future will hold, Banquo requests some prediction for his life too: this implies that he might put somestock in their ability to tell the future.  They then tell him that he will father kings; so, he will never...

At first, Banquo is a bit in awe of the Weird women.  He remarks on their appearance and actions, wondering if they are truly earthly or supernatural in nature.  After the sisters tell Macbeth what his future will hold, Banquo requests some prediction for his life too: this implies that he might put some stock in their ability to tell the future.  They then tell him that he will father kings; so, he will never rule, but his progeny will.

After they've vanished, Banquo wonders if he and Macbeth have just seen something true or if they've taken some drug that caused them to hallucinate the women and their strange words.  Once Ross and Angus arrive to tell Macbeth that he's been awarded with the title Thane of Cawdor, just as the sisters said he would be, Banquo begins to think about how strange this seems, and he grows suspicious that things may not be exactly how they seem.  He warns Macbeth that

"oftentimes, to win us to our harm, / The instruments of darkness tell us truths, / Win us with honest trifles, to betray ’s / In deepest consequence" (1.3.125-129). 

In other words, he fears that perhaps the sisters are motivated by some evil purpose and have told Macbeth one small truth in order to compel Macbeth and Banquo to believe bigger lies so that the men can be lead more easily to their own destruction.