IMO, Amazon screwed up on the K3's Audible integration: you can only download Enhanced format over wifi (instead of the more compact '4'), and it comes in one big audio file rather than in parts that you can manage separately.
The player interface is also kind of clunky. It is very awkward to back up to the start of a previous chapter, there are no bookmarks, and it is missing of the playback options normally found on my iPod ('scan', 2x speed, etc.).
I tried, really tried, to like using Kindle as my audiobook player. But in the end I just went back to using my iPod.