I currently freelance as a ghostwriter. Are there any other ghostwriters who have had any success?
I know K.A. Applegate was a Ghostwriter for "The Babysitters' Club" Series before she went on to write her own massively popular series Animorphs (which was the number one book series among Elementary and Middle School readers until unseated by Harry Potter). Her husband was a silent author (he helped write books but was never credited) and had his own successful series outside of the Animorphs series.
Well, I know Dean Wesley Smith is a successful author, with more than 20 million books in print, and he's done a fair amount of ghostwriting in his career. (He doesn't recommend it, IIRC.) I think his wife, Kristine Kathryn Rusch has done the same, and she's an award-winning author and editor. Both of them have LONG careers as writers.
Both of them are strong advocates of teaching people how to build successful writing careers, and both are what I would consider successful authors. They make a full-time career, and very comfortable living, with their writing.
One of the trickiest things about being a ghostwriter is not letting your identity get in the way. No one needs to know you've ghostwritten for someone else, but that also doesn't preclude you from having your own career as a writer in your own name/pen name. If others have done it, they may not say so, to keep their clients happy (and to prevent violating any NDAs they've signed), and so they can keep ghostwriting.