Horford is a very solid, and consistent PF/C. He rebounds well, scores well, defends the rim, and provides very good defense. He's better than any big man the Celtics currently have on the roster, by a wide margin.
IMO he isn't a max player unless he's being signed as a final piece to a championship team. He isn't the type of player you would build around. For this current Celtic roster, he isn't a max player.
Is he Dave Cowens, or Bill Russell in Tommy's eyes?...no.