I noticed that I finished the ebooks more often than physical ones and when I dissected my behavior, I realized that it was the heavy weight (e.g. 3 lbs) of the physical book that nudged me away from reading it. You can't hold a thick computer book in 1 hand. And propping it on your chest while laying down is also uncomfortable. Yes, the K&R C Language book is thin and light but a lot of books are 500+ pages and just too bulky to conveniently carry around and read anywhere.
For the few books that I need a physical reading experience to write notes on, I use the ebook and print one chapter at a time and then staple the pages. That chapter is then thin like a "magazine".
With ebooks, I've loaded the same titles on both the iPad and iPhone and can read a few pages anywhere while waiting in line.