I use playlists and they work fine for me. I haven't noticed any difference between 5.x and 6 on my 4s.
Here is my process specifically:
1. Buy a book from the audible website.
2. Download it using the plug in that audible supplies - the one that transfers it automatically to iTunes after downloading to my computer.
3. Make a smart playlist in iTunes with the name of the book and "plays = 0".
4. After the book has downloaded and been transferred to iTunes, plug in my phone, click on the phone icon on the left side panel (the one that represents your phone), click on the "books" top tab, scroll down to "audiobooks in playlists" (or something similar) and click the checkbox.
5. Hit Apply to have it sync.