I have used NetLibrary before to help customers, so I'm already a little familiar with it. I think it's easy to search for titles; however, I've found most audiobook downloading sites have extremely complicated instructions for downloading the software--and this is all before you can even get the book. I know this is for copyright protection, but it does make the process off-putting to many users. I think Project Gutenberg is less intimidating to new users, even though it has less material.
Downloading books is certainly a neat idea though, especially in this new ipod generation. I've never been a big fan of listening to books in general, but these days it's easier than ever thanks to equipment that can travel with you anywhere.
I am a big fan of ebooks, and that's one thing I did look at on the Project Gutenberg site. I've already read a few books this way (as an English major in college I often had to read books with expired copyright) and I've found it very useful. I also like the ad-free format of Project Gutenberg; it's very refreshing.