Here's a quick review:
It's a live CD that Mayer recorded with some blues folks. It has mostly new material, but does have two or three of his songs from his studio albums No Room for Squares and Heavier Things.
Anyway, this sound is very different from what you are used to hearing from John Mayer. Definitely not singer/songwriter with that rock/pop 'tweener sound and appeal.
This effort is blues all the way. Much more mature sound. He uses his voice a lot differently and really sells the blues.
I'm personally not into blues stuff but am a big John Mayer fan (gotta support my ATL boys) and am still overall pleased with this disc.
Quality wise this album is clean and balanced. If you are looking for a live, but clean blues album, I'd say check it out.