I've got the add-on rear diffusers on my factory bumper and I think it really makes the back end look a lot better.
The factory bumpers make my car look like an unattractive fat chick wearing a mini skirt. I do not want to see that much of the muffler, thank you very much.
They fixed it on newer models, but the 97-2000 or so models just don't have a rear bumper that goes low enough for me.