I have a love/hate relationship with nightcore. On one hand, it's not exactly...well, I guess you could say ethical. iirc, it's technically not legal under intellectual property laws, so there's that.
But at the same time, there are some songs that go from being good to being damn good when sped up and even pitched up a little.
Same goes for "Daycore" as well. The only daycore that I actually like is the slowed version of David Kushner's "Daylight" - which I believe was actually made by Kushner himself. Does help further it that because it's pitched down a little that it's a little easier for me to sing (I am a true bass and very recently was recommended to practice "Stars" from Les Miseràbles by my voice teacher)