Looking at Prime, there are a few free audiobooks that aren't tied to Audible. There are more free ones tied to Audible. What are the pros and cons of Audible?

I'm a very heavy user of audible (truck driver) for a couple years and I'm happy with it. Theres a very good selection in the plus catalogue which is included with your membership and you also receive a credit for any other book from the full catalogue (which is massive btw) each month. Beyond that I buy my credits in a 3 pack for $38 (it only offers you the 3 pack if you're down to your last credit) so anything cheaper than $12 I buy outright and anything over $12 I use a credit which is handy because I've seen some audiobooks for $35 or more.