No problems with eye train on my ipad. Is there any empirical proof that shows the ipad causing this?
I think I've seen studies both proving and disproving it.
IMO, it varies from person to person. I can't read for extended periods on a computer or iPad. It really causes me headaches. I have no idea why, but if I had to guess, it's probably the close proximity to a bright light. It doesn't bother some people, and it bothers the hell out of others. And yes, I know about brightness settings. On a lower brightness setting, my eyes struggle harder to read what's on the screen.
If not the light, then perhaps the screen glare could be causing problems for people.
Eyestrain isn't the only factor when comparing a real e-reader to a multi-purpose device like the iPad. On of those factors is weight. For some of us marathon readers, the iPad gets really heavy after a while. The nook and Kindle are super tiny compared to the 1.5 lb "beast". 1.5 lbs doesn't sound like a lot, but over time, especially when you're trying to vege out and relax, it can become a nuisance.
Battery life could be a factor too, but I'm not sure it makes too much of a difference. The iPad's 10 hours should be good enough for anybody, but some people go weeks, some even claim over a month, without needing to charge a Kindle or nook. That's gotta appeal to somebody.
So that's my take on it. This isn't Highlander. There can be
more than one right answer to different people's e-reader needs.