I find that the response time on my NookColor is fine; what is problematic is the touch sensitivity of the screen. Lots of time a "double-click" doesn't take right away or has to be redone to get a response. The screen size is an issue, too, especially when reading email, because the zoom controls are right in the same spot on the screen as the next page controls; the device sometimes toggles them almost randomly. I did buy a capacitive stylus, and that has helped a bit.
Reading with the backlight isn't a problem; I can do it for hours. It has a very wide range of adjustments available for the background color and contrast level, so I've been able to find settings that are easy on my eyes.
PS: I should note that the screen is perfectly responsive in "page-turning" mode; that isn't a problem at all. The issue is with web-browsing. It's a known issue that the screen sensitivity is a bit bad in the corners, and naturally that's where the software has placed most of the browser screen control "buttons".