I am a huge fan of John Mayer and have done a tutorial on Guitar Tricks showcasing his acoustic styles. You can find that at the link below;

http://guitartricks.com/inspiration.php?input=John%20Mayer

As for "Who Says?" I would agree it is not his best work but a good song. The lyrics I feel could be a little better but the playing is great and he sticks to a formula of sorts (which I feel is good at this stage in his career.) His new album "Battle Studies" will be released on November 17 of this year. Norah Jones will be releasing a new album as well on that day with the production of Jacquire King (Kings of Leon, Modest Mouse) featuring the guitar playing of the amazing Marc Ribot.