Auto-tune was originally intended to disguise "off-key inaccuracies", so vocal tracks can sound perfectly tuned despite being slightly off-key. Ha! Yes well no longer, it has become ubiquitous in pop music (crossing over to rock, r&b, hip hop) it has a sound all of its own. It deadens, flattens, makes the "singer" sound like an automaton, robot, at least to my ears. It can sound okay if it's done for affect, but used more than this, (especially if the singer uses it ALL the time) I will assume the singer either can't sing, or is always off-key. Imagine Janis Joplin using auto tune? Right, not a snowballs chance in hell. Imperfections, colorings, and inflections that a singer uses...will always win out over a device or a machine.
A rock singer of any merit wouldn't use it. But I've been surprised to hear certain artists like Maroon Five and Chris Cornell do. And yeah...I don't use it. But never say never right? (Though I have put a bit of flanger on my BR vocals or a bridge for affect!) So yeah confession: I hate auto-tune! It's used as a crutch far too often. I suppose in the same way reverb can be used, but ten times worse! Examples of singers who use it too much (Fubar, well that's another story hehe) Ke$ha, Nicki Minaj...
I look at it as a fad that will run its course, that in 30 years will be used again for novelty. Okay. There, I vented. LOL. Now I feel soooo much better...hehe.