Waterstones in the UK are running a survey on book and ebook purchasing. Apart from the usual 'how many' and 'what genre' questions there some interesting questions about a proposed subscribe-to-rent or subscribe-to-keep service.
I've no interest at all in a subscribe-to-rent service and I'm sure a subscribe-to-keep' option would be more expensive, for me, than my current 'buy-only-what-is-not-available-free' option.
http://www.voodoores.co.uk/ebooks/