The Great Wall is idiotic. It just increases barbarian defense by a 100%; if you have with barbs then you need to beef up defense. It also only outputs 2 cp/turn; much more worth it to build the Colossus, it gives 3.
I do like walls because they have no upkeep. In the early game now, I build lots of settlers, and need the Walls defense bonus. I've just got Rome to build many more settlers with the demise of my neighbor to fill in the gap, and the walls help.