I wouldn't call John Mayer a god, but he is an excellent guitarist. His own music is a bit too "pop" for me, but I was really impressed with him when I saw D&C. He and Oteil were playing off of each other all through the show. Their chemistry with the rest of the band was great. And he's not playing Jerry's riffs - Bob Weir specifically said that they didn't want an imitator; they wanted someone who knew how to jam. That's why they picked Trey Anastasio for Fare Thee Well, and why they went with Mayer for D&C.
Jerry had the most unique sound of any guitarist. Mayer may not be playing Jerry's "riffs" per se but he sure is copying his sound.