Vocals of metal are an acquired taste. Bands such as Judas Preist take a while to get used to.
Rob Halfords actually has a good voice. And the singing in Priest isn't
that different from "ordinary" vocals, such as Brian Johnson from AC/DC.
But I agree that many other metal vocals, especially in death/black metal, require time some time to get used to. I didn't like Dave Mustaine's and Alexi Laiho's vocal to begin with, now I love them both :)