I love audio books, if and only if the books are read by an outstanding narrator. Bad voice, monotone delivery, or over-the-top dramatics, and I pull the CD no matter how great the story may be.
I never substitute a great print read with an audio read; however, I've found that audio books do bring something to the table that I find valuable. I discover more in the work from a good audio narration. And, of course, audio books allow me to "read" while I'm out walking, or driving around doing errands, or even undertaking mundane household chores.
Listening to great writing helps me write. I hear the cadence of conversation more clearly, and I conceptualize the plot points differently than in a standard read. Nonfiction works bring their points home in a distinctly different manner than my standard yellow highlight method (mark it, review it later).
However, it's all in the narrator.
I've learned to skip any book read by its author. Few writers read their work well. Jill Conner Browne was somewhat of an exception, but I still think her Sweet Potato Queens work would be better served with professional narration.
I've also learned to choose audio books by narrator -- for example, I will listen to anything that Barbara Rosenblat has chosen to narrate. She's led me to the discovery of new writers, all because I trust her instincts to not read bad stuff.
Davina Porter is another great narrator; Jay O. Sanders is someone I like, too.
For a list of the top narrators in their field, check out The Golden Voices list. Award winners are listed, along with brief biographies being provided as well as complete audiographies.