I really love Rdio on the iPhone, I get access to a huge collection of music for $9.99 a month that I can stream or download (the downloading is the main reason I love it, aside from it being available in Canada that is). I can't really see myself paying for an ebook subscription though, my consumption of e-books is just different. With an ebook, I'm investing a lot of time into one item, with music I'm looking for quick experiences and a lot of them in short order.
I guess the closest parallel for me would be Audible for audio books. I had a subscription to Audible but ended up cancelling it after a few months, preferring to just buy an audiobook every once in awhile rather than paying a monthly fee to download one audio book a month. Aubible isn't a great example obviously since you don't get the option to stream their entire audiobook library.