Although his on-court impact will be judged next spring, we already know he will be a huge presence in the locker room. He is with his equals in the Big Three, which he never had (he was far superior to Kobe when they played, DWade was unproven, nobody on Phoenix ever won squat, and LeBron obviously did whatever he wanted).
Now he gets to be a (key?) cog on a team led by 3 other sure-fire Hall of Famers who have won a championship. Their is definitely mutual respect there, and that will show both off and on the court.
Plus, he is still a megastar who brings even more attention to the franchise and will most definitely be entertaining.