As an in-game manager he was mediocre at best. So it depends on how much value you're willing to give to the touchy-feely managing the locker room stuff. The players swore it made a huge difference. Naturally I tend to think that it's way overrated.
I will say this though -- the Braves were only able to keep that run going as long as they did because they were able to keep the core together. All three pitchers, Chipper Jones, Andruw Jones: these guys all re-upped without even testing the market. And that doesn't happen without Bobby Cox. There's no way to quantify it, obviously, but over the years that was worth a lot more games in the standings than if the Braves had had a better in-game manager who didn't inspire the same loyalty in his players.