*All quotes are taken from the Norton Shakespeare, based on the Oxford Edition.
In Act I, Scene 2, a bit of history is revealed. Ariel is reminded that Prospero's magic saved the sprite from a terrible situation (good). Even though Prospero believes this to be a good deed, the sprite is still being held against his will (evil). Before Prospero intervened, Ariel was forced into slavery by a witch.
Caliban, the witch's deformed son, is also under Prospero's command. Here Prospero uses his magic forcefully, threatening Caliban with it: "If thou more murmur'st, I will rend an oak,/And peg thee in his knotty entrails till/Thou hast howled away twelve winters." (296-298)
In act V, Scene 1, Prospero's magic has held the passengers prisoner (evil), but out of harm and out of the elements (good). Although the act of the tempest culd be seen as evil, Prospero never physically harmed any of the men.
The wreck has caused the passengers to endure sorrow for those they believe to have lost.
Ariel tells Prospero that the men's mourning could even affect Prospero: "Your charm so strongly works 'em/That if you now beheld them your affections/Would become tender." (17-19)
The good that comes out of the magic is that Prospero is willing to give up the practice once all the spells are removed.