All great points from Evo, whose Podiobooks.com site is trying desperately to change the face of how people get their audio content (although Audible.com gets all the press :)). In order I prefer: podcasts, individual downloads, audiobooks, ebooks, traditional books. My rationale goes almost entirely toward the convenience of listening. I like to listen while driving, or on the train, or walking. Very hard to do that with a traditional book. If I have the time to sit and read a traditional book, chances are I have the time to do something else, too.
When I have spare book money (like a gift certificate), I always go for the audiobook. The first thing I do is rip it, which results in about 100 MP3s or so (one small file for every segment on every CD). I then use my MP3->Audiobook converter to produce a single audiobook file, which I can play on my iTouch on "faster" mode. The most time consuming part of that task is not even the ripping - iTunes has got that down to a 1-click function. It's finding the files. Most audiobooks do not bother to put the appropriate ID3 tags in their files, so half the discs will go under the Author name, some under the Book name, a few under Compilations....and then sometimes the tracks are labelled just Track 001, or Introduction - 01... The biggest pain is in finding and sorting them all.
But it beats the heck out of carrying around 10 CDs!