I have had a very positive experience in traditional publishing, but I know many authors who have not. In my research life, having a prestigious publication by a highly regarded academic publisher has made the difference between long-term career stability and unemployment. Moreover, I was lucky to work with wonderful editors who not only helped me make the book infinitely better but who also provided important guidance in building my career. Cornell University Press was committed to my book Code Green: Money-Driven Hospitals and the Dismantling of Nursing, provided a gorgeous cover, and they continue, even ten years after publication, to promote the book in their list. Should I publish another book based on my research (I've tended toward articles), I would be hard-pressed to consider self-publishing.
My newest fiction project is another matter entirely.