There have been some good voices, but the problem with AI really is that the format is such that it really is disinteresting in comparison to something like "The Voice" (which never seems to get a thread here)
The reason why I say that is that on AI, the critiques rarely provide the audience with any real insight and the interaction between the judges and contestants are minimal; in comparison to the concept of a tutoring program to strengthen the competitors.
This year on AI, they've done more of the coaching and have shown it, but it's surprisingly bad, and as often as not competitors just ignore what they are told.
You're right about some good voices..
I also have to say, Mariah Carrey is a HORRIBLE judge. After previously pointing out how many Beatles songs she has never heard (She's Leaving Home, etc.) she went on this week and said she had never heard Garth Brooks "The Dance". I'm not even a country music fan, but damn, that song has been on a ton of soundtracks and commercials, a superbowl ad.. I mean, it's not that obscure.