I've been looking into it for audio books for DD - I don't do audio as it goes in one ear and out of the other....
They have lots of unabridged kids' books. They also had a great deal on ipods 60GB when I looked but I only looked at the US site, not the UK one. i.e. .com rather than .co.uk
I'm not sure I'd want a monthly plan though.
I've listened to some of their samples and they are pretty lengthy!