The primary use for my iPt4 is audio. I don't read ebooks on it because it has an LCD display and the display is too small.
I don't borrow ebooks on it either because the OverDrive app doesn't support Kindle books and I don't read ePub. But I occasionally use it to borrow MP3 audiobooks.
Since the vast majority of library audiobooks are in WMA, those are the ones I checkout most. The problem is, they're not supported by the OD app. So, I have to use my PC for them. It's a lengthy process too, because the files have to be converted from WMA to some Apple format for playback on iDevices. For example, "2001: A Space Odyssey" took 11.5 minutes to download to my iPod.
So in the end, it's pretty much a useless app for my iPod. However, it's a great app for my Android tablets, because I can check out Kindle books for my Kindle 4 without a computer.