I've only had experience with the iPad as an ebook reader. I wouldn't recommend it if you aren't in the market for one as they do cost quite a bit more ($600) than a device that is just for ebooks. If you were in the market for an e-reader and a basic laptop it would be a candidate, but it doesn't seem like that's what you're looking for.
I can tell you that after reading 2 books on the iPad that ebook readers are awesome and I find myself much more likely to read now.
The Kindle is what I've heard most about. I believe it just experienced a price drop thanks to the iPad (I want to say $200). It has an e-ink display, so it won't have annoying reflection in sunlight, and it uses almost no battery power. You download books directly from Amazon on the device. Prices are generally slightly lower than the paperback version I believe.
There is a Kindle app as well, so if in the future you get an iPod Touch, iPhone or iPad all of your kindle books will just show up.
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