Heavy backpacks are a big concern for undergrads because their classes are often spread throughout the day all over campus, and eBooks cumulatively weigh less than one pound, instead of five or more pounds each. However, heavy backpacks may not be as big a concern for grad students. Your classes are probably clustered in the same area, or building, on campus, some days you may only have one 3 hour night class, and you may even have a shared Teaching Assistant/Fellow office on campus where you could switch out books during the day.
While eBooks can lighten your load, they can also be harder on your eyes. In grad school you have to read for sustained periods to complete readings on time. However, reading on an electronic device for long periods of time can cause Computer Vision Syndrome, which includes eyestrain, headaches, blurred vision, dry eye, and pain in the neck and shoulders. These symptoms are more likely to develop when you read on an iPad, laptop, or multi-functional eReader (like the Kindle Fire) than on a dedicated eReader using eInk and a matte screen (like the original Kindle).