My 14yo loves listening to books. Always has. He can't get enough of them, re-listens to his favorites over and over. He listens while building legos or drawing, he listens while just hanging out. His working vocabulary was impressive at an early age, due largely, I think, to his hearing the words in use from his audio books. When he was 8 he was correctly using words like "affronted" and "resolutely". His syntax varies depending on his current favorite author as does the rhythm of his speech. He currently is sounding like Bill Bryson, and what is even funnier is that his older brother, who hasn't heard those audio books, is picking up on it and now is sounding like Bill Bryson too.
I really enjoy audio books, too, though I'm not able to listen for the endless hours like my son. I like how a good narrator can bring a story to life with different voices for the different characters, or how their phrasing can clarify the meaning of a sentence or paragraph. I like that hearing a book aloud, or for that matter, reading a book aloud, prevents you from being an impatient reader and skipping past the descriptive paragraphs or tedious expositions to get to the action.
The only problem with them is that you can't dog ear a page, or underline a particularly striking sentence. You can't stop and look at how that striking sentence is constructed, or stop to think about it's deeper meaning -- the narrator just carries you on to the next page, the next plot point.
I just finished listening to Northanger Abbey, and found myself laughing aloud quite often. The narrator was fabulous, but I need to get a print copy and find some of those sections that made me laugh. For instance, there was some very astute satirical commentary by Austen -- I don't think it was in the dialog -- about the perceived attractiveness of women who act dumb, or are dumb or stay uninformed on purpose. I was driving the car and laughing when I heard this bit, but I need to find it again to get the quote right.
I'm also reading aloud The Lord of the Rings to my 14yo, and am amazed at how much more I am getting out of the books by reading them aloud. I read them when I was a teen, but I've only re-read my favorite sections since then, and I had no idea how much I was missing such as plot points and characters. All that geographical detail still bogs me down, though! We're having a good laugh, too, over the biblical sounding language in the 3rd book. "And lo! Aragorn did crush the athelas. And the scent filled the room filling all hearts with gladness." Or something like that -- it gets to be a bit much!