Personally this is my favorite line of the entire play.  I wait on pins and needles until it is uttered.  This is not an unusual reaction.  Shakespeare designed these four lines in the play with great care, and the dramatic tension is designed to culminate in this exact moment.  Macbeth, due to the prophecies given by the witches, has been made invincible.  They have told him he cannot be killed by any person born from a woman.  To an audience member, this announcement guarantees Macbeth's invincibility.  Everyone in history has been born from a woman's womb.  

Two literary techniques are at work here.  The first is situational irony (where the audience believes that the story is going to go one way, but an aspect of the play changes it to go in a new direction). The audience is certain that Macbeth will win this duel.  Shakespeare reinforces this point with the death of Young Siward, who he dispatches easily.  The plot switch, whereby Macbeth becomes vulnerable to death due to Macduff's cesarean section, changes the path of the scene.

Secondly, the lines employ dramatic irony (where the audience or a character has secret information that is unknown to the characters in the play).  Both Macbeth and Macduff have information that the other does not know about.  Macbeth's revelation is expected.  Macduff's piece of dramatic irony is unknown to everyone and creates an emotional ah-ha! This, coinciding with the sword fight that leads to Macbeth's death, is the climax of the play.  

This quote helps emphasize how Macduff is a foil for Macbeth.  The quote shows the strong contrast between two characters.  No scene in the play provides a more riveting contrast between the fallen hero, Macbeth, and the true hero of the play, Macduff.  

As for the poetic devices used in the quote, the lines are written in blank verse, which means that there is no rhyme scheme.

Shakespeare also uses the paradox (a contradiction of what is said and what is meant) referring to Macbeth's "Angel" that he serves.  More than once in the play the characters refer to Macbeth as the devil or serving the devil.  The use of the phrase "angel whom thou still hast served," is paradoxical because the belief is that Macbeth serves the devil and not God.