Firaxis leaderheads are, for the most part, better. Some of their units (less than half) are superior to just about anything here, in terms of animation (although some creators' here come pretty close). Their terrian sucks, and their skills at manipulating the editable game mechanics are poor. Some of our mods here are better than anything Firaxis made, and the knowledge base of this community as a whole seemingly surpasses Firaxis. I say seemingly, because perhaps they locked the one guy who studied the mechanics of the game, knew history, watched the AI, and could do arithmetic, in the closet and didn't release him until after the game hit store shelves.
No hard feelings to any graphic creators here, but let's be honest: Civ3 was made by professionals, and despite how much we'd like to believe our own efforts or those of our friends result in better work, it is not always true. But we shouldn't be concerned when fan art isn't quite as good as paid professionals, because the hundreds of graphics here flesh out areas of the game Firaxis never got around to making (white Zulu's, anyone?). And in many cases, the difference is marginal.