I had much the same reaction when I was living in Memphis and working in their public library; some people had such thick accents that I couldn't understand them, even when I asked them to spell out words, while others had less of a Southern accent than I do. (I'm originally from central Illinois, where a lot of people still retain the Kentucky hill accent that people like Abraham Lincoln's family brought when they moved west in the early nineteenth century.)

And, yeah, writing in that sort of phonetic/accented dialect is almost never a good idea. (See also: Joel Chandler Harris, H.P. Lovecraft.)