I started using NetLibrary and Overdrive a couple years ago. So, for about 2.5 years, I have spent quite a lot of time downloading and listening to the ebooks. I have done quite a lot of troubleshooting and training with customers and some of the staff on the use of both services as well.
Neither of the two services is compatible with the IPOD. The player must be PC-compatible. I've heard people saying that there are some PC-based players that don't work with the services, but I don't know which ones to avoid. I haven't had any trouble at all with the two I have owned (both manufactured by Creative Labs).
Of the two services, I prefer NetLibrary. Overdrive's downloads are usually comprised of multiple parts. You need to use software downloaded from the site to manage these files. Also, there is a lot of competition for access to the to books. Overdrive only allows a certain number of concurrent users per book, based upon how many concurrent uses have been purchased from the vendor. Okay, I understand this - but it's not a physical BOOK!
NetLibrary allows unlimited usage of its 1600+ titles. (When we started, there were just under 1000) Wonderful. And the file download is managed by Windows, and played by Windows Media Player. Easy. Now, for those of us who are using MP3 Players with "Flash Memory," Netlibrary can be a bit harder to use, because the book is contained within one continuous file. Ideally, one should use a player that allows bookmarking. Otherwise, if you should choose to listen to anything else on your player, you are forced to remember where you stopped, and to fast-forward eternally until you find your place. This is very wearing on the battery. If you replace the battery, you may have to hunt for your place all over again. Sigh.
Since I have been a NetLibrary user for about 2 years, I spend some time with Project Gutenberg for this "Thing." Project Gutenberg's Audiobooks project is an excellent resource for a student who is short on time. However, I have to say that I'd need to be desperate to use the audiobooks that are read by a computer (rather than by a person). My mind kept wandering during the reading of Tom Sawyer. I tried "The Little Match Girl," which is one of the 273 "human-read" files, and was pretty impressed. It wasn't a dramatic reading by any stretch of the imagination, but still, the reader was better than one author I once heard who insisted upon reading her own book on tape. I'd certainly recommend these to anyone who is pressed for time, or perhaps to teachers of students with reading disabilities.