Magic is one of those things that can have either positive or negative consequences, depending on the intentions of the one who yields it. The wand, according to Rumple in ep. 4, was “pure evil” but I think he said that because he already had a vendetta against fairies. Of course, we do know from ep. 3 that evil fairies exist and that their fairy dust is harmful. I think that there can be either negative or positive consequences to magic depending on the moral makeup of the one who grants a wish. So far, only Rumple says, “all magic comes at a price.” It is in his interest to say so, since he gets to make a deal in his favor. I think in that case of the Blue Fairy, however, she is righting a wrong, so the consequences are favorable, like Jiminy becoming a cricket to help Gepetto.
"That’s how you know you’ve really got a home. When you leave it, there’s this feeling that you can’t shake. You just miss it." Neal Cassidy