Perhaps my greatest concern for e-reading is "how does it affect my ability to fall asleep when I read in bed?". A 10" iPad puts out way too much light even at the lowest setting and keeps me awake. An e-ink device works much better because it uses indirect light. However, I've found that my 4" phone works best of all. On its lowest setting it puts out very little light. And surprisingly 4" is also a better form factor for reading novels. The small screen reduces the amount of eye scan needed, and the small size also allows a very large variety of comfortable reading positions.