M asked me to make a more lengthy case for why the Kindle rocks, so here goes:
1) CARRY YOUR WHOLE LIBRARY WITH YOU WHEREVER YOU GO! For non-bibliophiles this may not seem like a big deal, but if you're a book whore like me you know that sometimes you just have to, have to, have to re-read Chapter 4 of your favorite book when you're on the bus/in an airport/at work/somewhere not home where you favorite book happens to live. With a Kindle, not only can you read that chapter (or the whole book) anywhere you want, but even if you didn't bother to download it before you left home that day you can run to Amazon (or a wide variety of other eBook retailers) and buy it no matter where you are. Well, no matter where you are in the continental U.S. International Kindle book buying requires a few more steps and access to a computer, but that detail won't matter to most American readers. The added bonus for those who travel frequently is that you don't have to pack a ton of books or be left with nothing but the in-flight magazine to read if you run out of reading material.
2) E-INK TECHNOLOGY! This really is see-to-believe stuff. Reading a Kindle or other e-book reader is no different from reading a paper book in terms of eye strain. The e-ink technology isn't anything like a computer screen, so even if staring at a monitor for hours on end isn't your thing, it should be a non-issue with a Kindle. You can only buy Kindles through Amazon, but Target carries the Sony 505 Reader and should have one on display if you're interested to see e-ink in action. It's very cool.
3) MAC COMPATIBILITY! Frankly, I probably would have bought a Sony 505 if it had only been compatible with my beloved iBook. Sadly, Sony's readers (which are cheaper than the Kindle, though you can't download books onto them without the help of a computer) only work with Windows. I love my Mac, I will always love my Mac, so the Kindle was my best eReader option.
4) CHEAP(ER) BOOKS (MOSTLY)! There's a lot of debate in the e-Reader community (yes, I'm a member of the e-Reader community now, yes Juan has mocked me for it) about the price of ebooks. Frankly, they're not much lower than the price of most paperbacks, and in some cases the price of the ebook is actually higher than you'd pay with a discount card at, say, Barnes & Noble. Still, if you shop smart you can still usually pay significantly less for your ebooks than you would for the paper versions. When you're plunking down $$$ for the reader in the first place, it's nice to know that eventually you'll make up the cost in book savings.
5) REDESIGN! The original Kindle still has a lot of fans--people love how distinctive it looks, how you can expand your book collection by using an SD card (not available with the Kindle 2), etc... The Kindle 2, however, which I have, is an improvement on the Kindle 1 in several ways. The most significant from a regular user point of view is the fact that it's now very difficult to accidentally turn the page because Amazon redesigned the page turn buttons. Turning pages on purpose requires next to no thought, but you aren't going to accidentally flip through several chapters and lose your place the way you could with the Kindle 1.
6) ANNOTATION, BOOKMARKING, TEXT-TO-SPEECH, ETC...! Just like with a paper book you can make margin notes with a Kindle (you can't with any of Sony's readers), you can bookmark several spots in a book, and unlike a paper book the Kindle will read your books TO you if you choose. This last feature is very controversial and may eventually be phased out because of copyright/legal issues, but for now you can use it on most ebooks. Given that the price of audiobooks is hideously high, text-to-speech technology rocks since the cost is just that of the ebook itself.
7) DID I MENTION PRETTY?! Yes, see previous post. The Kindle is pretty.
8) LOSING SIGHT OF THE DEVICE! I've now read two average-length novels on my Kindle and can verify that, at least for me, the device doesn't prevent me from slipping into the story the way I do with paper books. Some people argue that the white color of the Kindle makes this difficult, but so far it hasn't made a difference to me.
Bottom line--I love this little machine. If you're on the fence about buying an e-Reader, I would strongly recommend checking out this forum for the scoop on a wide variety of e-Readers, including the Kindle.