Comparing Sanders to McGee is off the deep end. There is a lot more than shot blocking that goes into defense- Sanders had such a huge impact on the Bucks defense last year that there were whole academic studies dedicated measuring how much he deterred people from even attempting to drive to the basket or shoot close in. Last season Sanders did something on defense that JaVale McGee has never had any idea how to do, and it had to start with playing his position well. He knows how to play defense and he has the gifts to do it with Dwight Howard-esque effectiveness.
I agree, though, with you guys who are scared of his off-the-court craziness. It would be foolish to invest 4 years in a player who gets himself hurt in night club brawls, unless you had strong indications that is was over and done with.