Kevin Spacey's accent is pretty darn authentic. As a native South Carolinian, I have a few minor winces with it, periodically, as does the author of this (excellent) article/video on linguistics. But overall I give it an A-. Most actors deserve Fs for their southern accents.
Southern accents, and their finer or lesser points, are (were) absolutely critical... to the southerner at least. As they instantly denote whether or not there has been any education in one's family background.