The most informative survey was conducted recently by Marketing and Research Resources, Inc.--but paid for by Sony, a pioneer of e-book readers and a company with a considerable stake in the results. Luckily, the results aren't Sony-skewed--it looked at 1,200 e-book reader owners, including those who own the Amazon  Kindle, Apple  iPad, and Sony Reader.
The survey found that 40% of e-book reader owners say they read more than they did prior to the purchase of the e-book reader. 55% say they read the same amount, and only 2% say they read less. Amazon has said that Kindle owners purchase 3.3 times as many books as they did before the purchase of the Kindle, and e-book sales have risen along with sales of the Kindle (and other e-book readers). That allows authors like Seth Godin to go digital-only --there are still lots of readers out there in the digital world.
A survey from the Book Industry Study Group found an interesting gender reversal as well. Though women make up the majority of physical book purchases, 52% of e-book purchases are made by men. It's suggested that this is due to the early adopter gender spread: Men tend to be early adopters of tech more than women, and by some accounts, e-book readers still qualify as early products.
Anecdotally, I agree wholeheartedly with the findings. I've bought two books in the last week (the new Gary Shteyngart is quite good!) for my Kindle, though had I not owned the e-book reader, I'd likely have gotten it from the library or possibly just watched Top Chef instead. E-book readers, in whatever form they take, are good for the industry even as a fledgling medium.