The slums looked good because they were real slums, the occupants were being moved out as the filming was taking place - as it happens they were being moved to "better social housing" (aka District 10).
Oh, and all the animal parts hanging up in the slums? They were real to, whatever was in the shacks when they opened the doors are what was in them when they were occupied (with the exception of the one fake slum above the ship).
Edit: quite a bit of the IMDB "Trivia" section is actually a good read for this film, it's not normally, and has some interesting facts.
- The film is based on the directors time in Joburg, he grew up during the apartheid
- All the shacks were real, and the residents were being moved out in parallel to the filming
- Animal carcasses were as found in the shacks, with few exceptions, and left in the film
- The "unreal" looking sniper rifle was real, made in SA
- Sharlto (Wikus) adlibed all of his lines