I've become addicted to them. I think they're the ultimate in media - the length of a book (20-30 hours of listening, typically) that doesn't require your full attention to enjoy. I listen to them during my commute, while exercising and while doing most household chores.
The can be expensive, but I by mine used or download from www.audible.com (2 free per month for 20 bucks). I only buy on CD and rip 'em to MP3's to listen.
I've put away 21 in the past year alone...that more than doubles my reading output during that time period.
Highly recommended, especially the extremely well-done Harry Potter books.