First, I love audio books, or at least most audio books. I have checked out audio books on cassette tapes and cds from my local library for my children to listen to when we traveled since they were very young. They were introduced to Ramona, Henry and Ribsy to begin with, and eventually we made it through Heidi when my eldest was only in first or second grade. I also found them to be a great way to revisit a story we have read aloud: Reading aloud at bedtime, it took us a month or so to get through one of the early Harry Potter books, so that by the end they didn't remember all the details from the beginning. Several months after we finished it we listened to the audio book while travelling and they were able to put it all together better this second time. The obvious downside is that not all readers are good, or sometimes they simply don't fit my idea of how a character/narrator should sound.
These days I still check out cd audio books from the library, but I usually rip them into iTunes for portable listening. Mercer County libraries have a limited selection of playaways, but I have used them on occasion. They work well, and can be used in my car with the same adapter I use for my phone/ipod. The down side, of course is that it's one more gadgety thing to keep track of.
I have also purchased audio books through iTunes, often of titles I think my children will enjoy and/or that I need but are unavailable from my library.
I have looked into Listen NJ several times, but as the owner/user of an iphone for my audio, I have generally been dissapointed. I see that it is now possible to get some titles for the iphone, but the selections of available titles is quite limited and rather uneven: There seem to be more science fiction than general popular fiction, and with only one Jane Austen title available in the required format.