I hear it all the time from a variety of librarians: not going to invest in e-books until I know they will get used." In fact, I hear it most from school librarians who are low on budget and don't want to invest in e-books without proof: from someone else's school system, that the investment will payoff. Luckily, usage data is available for e-book publisher and aggregator products, and this data can be mined in a variety of ways to determine use of the product or particular titles.
(2009). E-book Usage Data. Booklist, 105 (15), 75.