I've done a ton of research on this, and I think I've talked myself out of getting a reader.
The Ipad is considerably more expensive than the Nook; and from what I've read, the Kindle and Nook are much easier to read for long periods of time than the Ipad. The Ipad also runs much warmer, and doesn't do as well outside in sunlight.
Perhaps the main reason I was looking at getting a reader is that I seem to be downloading a lot of pdf files that would be nice to have in a reader to take out on the porch, or to town when I go eat, or take to bed for late night fall asleep reading, etc., and that would be easier on the eyes than a regular computer screen. However, readers do not handle pdf's very well. So I've played around with a converter that will convert pdf's to epub; but it still does a less that adequate job. Here, the Ipod, from the accounts I've read, does a much better job, as it displays pdf's properly.
As far as the Nook and the Kindle, I found the Nook to be the better choice for me, if I were to get a reader. (The Nook is Barnes and Nobles' reader.) The Nook has wifi access, so if you're in a restaurant with wifi, you can access and browse the web. The Nook has a replaceable battery; and it supports Epub. And it's only $149. And to do conversions for the Kindle, apparently you have to email your pdf to Amazon and then download the converted file, rather than just do the conversion yourself on your computer.
But I think I'm just going to pass for now. Since none of the ebook readers can handle pdf's adequately, and since pdf's apparently do not convert well, I just don't think a reader is going to work well for me.