Justin Timberlake. I think Justin's music is simply terrible and has no creativity; there's just no variety in his songs. The lyrics don't make up for them with their dullness, and his voice makes his songs even more grating to listen to. I don't see what's so great about him.
John Mayer. I can see why people may like him, but I find his songs particularly irksome because of the lyrics. His voice sounds distorted as well. Overall, his music is mediocre to me.