I'm a fan.
I learned to appreciate the blues because of John Mayer's music. It served as a transition from the usual pop/rock music.\
During the "Room for Squares" album, I thought he was just another Lifehouse, Dave Matthews-sounding artist. And "Your Body Is A Wonderland" has gotten me a few places too.
When Continuum came out, I was surprised with his new sound. That's when I started listening to the albums between Continuum & Room for Squares.
His Room for Squares album, I think, served as a way for him to be introduced to a lot of ears. I like how he turned away from pop, & went on to play his own style of blues. I hope he doesn't sell out - I think he's doing fine as is.
As a guitarist, I think he can really go a long way. There's a lot of soul in his playing. He's very young compared to Clapton & the other blues greats, so I look forward to what he can do in the future.
Let's just say that he also influenced my choice on what kind of guitar to play. (check avatar)