I think Brad overall has been a very good GM. He's made some outstanding moves (trading for Al, White, Brogdon, at minimal cost). I don't think the Muscala trade worked out, but then again, what we spent for Muscala (second-round picks) is so minimal, who cares.
I think his only significant mistake so far was extending Mazzulla prematurely. I get it: he wants to show support and love for a guy who stepped into a difficult situation, and remove uncertainty hanging over him. But the guy's a rookie coach for an elite team; he probably should have some uncertainty hanging over him. Also, Brad has built a team that is supposed to be competitive in the playoffs for the next, what, four or five years anyway? Why not see what kind of playoff coach Mazzulla is before extending him?
By the way, lest this be misconstrued: I'm not even saying Mazzulla is a bad coach, or they need a different coach. I'm just saying it was too early to extend him. I know, they can still can him after the season if they're not happy (I doubt Brad would do this on his own, but might get pressure from ownership), and it's just money down the drain. But there would be bad feelings too. I don't know why Brad didn't wait.