my sister is a huge john mayer fan and she thinks he is INFP... (she reads his blog and website etc)
-though his propensity to "talk to his fans" both inbetween songs, responding to email, posting tons of blogs etc sounds very ENFJ
-his ability to practice such a Se activity over the years seems very ISFP
-his lyrics are markedly N
-he does funny things (like respond to reporters in Japanese, to get a laugh)
-his machevialian way of being a fake pop star, before coming out with his true blues, could be every ENXJ of him....
INFP and ENFJ are mirror types arent they? maybe something goes on when he tries to introvert as an ENFJ...or maybe something goes on when he tries to extrovert as an INFP.