I think it's like horror movies, it needs to feel real to be any good. If it sounds like the singer genuinely went psychotic and could actually murder you or bomb a hospital or something, then I can get it. When I heard Corey Taylor on Iowa for the first time I nearly shat myself in horror. I can appreciate that. Unfortunately, once it starts sounding too practiced, controlled, stylized and genre-appropriate I just lose interest completely. Especially when they turn 180 and start singing clean in the chorus, it's like watching a bad actor who can't make up his mind about what emotion his character is trying to express. Lastly I think the rest of the production needs to gel with the vocals too. For growls to sound sincere I need the drummer to play faster than their ability allows, the guitarist to bang their strings out of tune and the bass player to knock over a cymbal stand in sheer rampage. If it's all neatly produced and tight and music collegey then it just doesn't make any sense for me to have vocals like that.