I'm pretty familiar with both Overdrive and Netlibrary, since I attended one of the 'Downloadable Audiobooks' trainings we had in the system. Both are relatively simple to use and navigate. I like that some of Overdrive's titles can be burned to CD.
Unfortunately, I haven't actually downloaded an audiobook. I own an iPod, so my portable device will not play the .wma files that these books are formatted in. And if I'm near my computer, I'm usually doing enough multitasking that I can't add "listening to a book" to the list.
But, any time I've tried to help a patron find a book on Netlibrary or Overdrive, I've been very happy. Especially the classics, since it's usually a kid that should have read the book 2 months ago but just remembered he needs it tonight for the project due tomorrow. If you're comfortable enough with technology to be asking about audiobooks, then you'll be comfortable navigating their sites. The rules about "permissions" and "licenses" are a bit hard to explain face to face, but I think once people download all the bits to play the files, they don't really think about it all too much. They just know the file will "expire" in 3 weeks.
I think downloadable audiobooks were a great addition to the services we offer. I just wish Apple would share their iPod code so that 3rd party distributors could create compatible files. I hate the looks on patrons faces when I explain they are not compatible with iPods.