Disadvantages:
1. With audio books, rewinding/revisiting an older chapter/section is a bitch.
2. It sorts of curbs your imagination.The intonation and the modulation of the speaker is critical for determining the visual setting of the story. It is like acting in a movie being directed by someone else - you have little control over the creative process.
3. The reader's speed might not be compatible with yours. I can read upwards of 300 wpm while maintaining reasonable amount of retention and analysis. Audio book readers, on the other hand, are slow to accommodate people of various reading speeds. Hence, a book that takes you 6 hours to read might take 12 hours on an audio book.
4. Reduced reading efficiency. While reading an actual book, you are engaging your eyes in the process along side your auditory sensors (if you read out loud). I find that I remember things I see better than the ones I listen to.
5. You might need a little time to the reader's accent.