Audio Books: What do you like, and why?

Amazon, Play, Audible, Borders.

I've become a huge fan of audio books recently, for quite a few reasons.
In my view, they sit in between watching a TV/Film, and reading a book, but have the ease factor of watching TV, combined with still stimulating the imagination of a book.
Plus over the past 10 years or so they seem to have really come on in leaps and bounds with special effects.

I'm generally a huge fan of Sci-Fi stuff, and have quite a bit of Dr Who (recently got a great original Doctor box set, introducing the Daleks!).
Also some of my favourite Star Trek books have been laid down on CD, and as for Star Wars... Thrawn anyone?
Stephen fry also did an awesome job with Harry Potter, but then he is a god in his own right :)

For me, i love listening to these books on a long drive, or in 30 minute stints travelling to work each day.
What do you guys and gals listen to?