BobR,
I have been an audible member for about 18 months now, and I think it is a fanatastic deal!
At the 2 book a month level, you get your choice of ANY of their audiobooks for an average price of $11 each. I've purchased books listing for 10 times that price as my monthly selection.
They do have their own format, rather than MP3. They support 100's of different MP3 players. I currently use both my Palm TE and a Sandisk Sansa MP3 player with Audible books. You can also download the books to your PC and burn them to CD's, or play them using Windows Media Player, or, I think, Itunes.
They have sales several times a year where you can get about any book on the site for $10-$12, sometimes less.
They seem to stay fairly current with new offerings.
I know, I sound like a commercial for them!
I have no affilliation with Audible, other than I COULD send you an invite to join which would earn me a free book if you did. *grin*
All in all, I've found Audible.com to be a tremendous deal!