As long as you get something with an e-ink screen, you'll have no problems at all. Ordinary screens are back-lit, and refresh 60 times a second or more. You're staring at a bright, flickering thing all day long. An e-ink screen is much more natural. It's not backlit, and the picture is only refreshed when you push a button, such as to turn a page. It actually works a lot like one of those etch-a-sketch kids toys, on a more impressive level.

So yeah, you get no more eye-strain than you would reading a regular book. The words stay still, and the only light you have is whatever comes from your environment. The other benefit of not being lit and not refreshing all the time is spectacular battery life. On a typical screen you'd be lucky to read for a few hours. With an e-ink device, you're likely to finish several books before you need to charge. They advertise around 10,000 page flips, and the trickle charge needed to power the buttons and whatnot is very small. It will standby for months if you don't use it.

You should get one. I've personally tried the Sony PRS-505 and 705 (think it's 705? the newest one, anyway) and the kindle, and prefer the 505. The kindle is plastic, and doesn't feel solid in your hands. It's not very natural to hold. The 705 is thinner and newer than the 505, but it has a touch screen, which is actually bad. Whatever coating they put on to make it touch sensitive, or protect it from finger oils, or whatever it's for, produces some serious glare, and the text doesn't look as good. Anyway, I'd get the 505.