I think the Raiders will end up being very happy with MM, and just as importantly, the Raiders and Josh McDaniel are probably a really good fit for him. Mayer certainly isn't going to win any sprinting contests, but I was continually amazed with his understanding of how to catch a football. He was incredibly smart at being able to use his body to screen off the defender, and on top of that he has absolutely phenomenal hands. If it was going to be a contested catch, you felt pretty good about the odds of MM coming down with the ball.
As for comparisons, I sort of liked the "Baby Gronk" nickname, but as others have pointed out, Gronkowski was a little bigger than MM and probably a little bit faster. I really like the comparison to another Raider great and former Domer, Dave "The Ghost" Casper. Casper was a big guy (at least for his time), but more than that, he was also a really intelligent football player. He knew how to get open. He knew how to find the open spots in the zone. And he knew how to body off the defender to come down with the ball. That pretty much describes Michael Mayer's game. If MM can have the career that The Ghost did, the Raiders will have a good one.