I've been a loyal customer of both Amazon and Audible for years, and even more so since I got my Kindle. (It's like it was all part of some kind of plan...)
But having a membership at Audible can be tricky.
As a Gold member, I pay $14.95 a month for 1 credit. I also get a 30% discount on all other purchases. That right there is worth the price of admission, especially considering that for my $14.95 credit, I can download a book that costs much more than $14.95.
Example: Stephen King's Doctor Sleep, which is worth 1 credit, is normally priced at $34.99 (although members can buy it for $24.49). I paid: $14.95.
Now here's where it gets tricky.
The pricing is such that not all audiobooks cost $14.95.
Example: James Ellroy's Hollywood Nocturnes, also worth 1 credit, is in my wish-list. The non-member price is $13.21 while I can pick it up with my membership for $9.25.
If I were to use my monthly credit to pick that one up, I would have paid $14.95 for something that would otherwise cost me $9.25. That's kind of why that one is still in my wish-list rather than my library.
Here's another trick, although at the opposite end of the spectrum. Since Audible is an Amazon company, they are able to offer combined deals that could result in getting more for less: Literally.
Example: If I were to buy the ebook version of Lawrence Block's The Burglar Who Liked to Quote Kipling for my Kindle, it would cost me $4.74 and I would have the opportunity to buy the "professional narration" (ie, the Audible audiobook) for an additional $2.99. Total cost for both ebook and audiobook: $7.73.
Just buying the audiobook from Audible would cost $18.11 with my membership or, if I used my monthly credit, $14.95, and with that option I wouldn't get the ebook version with it.
Buying both would save me seven to ten bucks!
Long story short:
A) Don't waste your credit on anything that costs less than $14.95.
B) If the ebook/audiobook combo is cheaper than $14.95, buy that instead.