I like my Kindle e-book reader. It's handy for checking out library books, and I like that I can use "Manage my Content" at Amazon.com to return the library books as soon as I finish them, to reduce wait times for other people. The Kindle is good for inexpensive, out-of-copyright, classics, even though sometimes the OCR is of poor quality. I don't read so much that the Kindle reduces the weight of my luggage significantly (more's the pity). The Kindle makes it very easy to buy more books than I would otherwise, which was surely a design consideration.
But, the "killer app" for an e-book reader, for me, is foreign-language books. More often than I like, I need to consult a dictionary to translate a word I do not know. When reading on the Kindle, this becomes extremely easy: just move the cursor onto a word, and the dictionary definition pops up at the top or bottom of the page. No longer do I need to interrupt my reading to page through a dictionary, then find my place again. This has transformed my reading by making it easier and more enjoyable to read books that are slightly beyond my reading level. You need to buy a translating dictionary; installing it is an easy process that is available from the Home menu under "Menu > Settings > Change Primary Dictionary".
Many dictionaries have some problems with word forms, such as the frequent case when a pronoun or article is appended to the end of a Spanish verb. If navigating to the word doesn't bring it up, you can always type the word into the dictionary search, but at that point I usually just keep reading. If I do choose to type the word, another problem is that you cannot type accented characters on the Kindle keyboard. Instead, I type the word without accents, then go to the list of all nearby words and select the accented form from the list.
The main downside of the Kindle is its slow display refresh. This is completely tolerable for turning pages, but is irritating in navigating menus and other activities. I do notice it when navigating the cursor to the word I don't understand: the definitions of other words along the cursor's path flash at the top or bottom of the screen. Slow refresh also makes it hard to judge when I should let up on the "5-way" joystick controller. The forthcoming Kindle Touch may solve this problem by letting you touch the screen directly rather than using the somewhat clunky "5-way" joystick.
I notice that battery life is a bit lower when using a translating dictionary, probably because of all the display changes that occur as I navigate to the word. But, battery life is still great: the Kindle battery lasts weeks.
Update, October 2014:
I have now read many Spanish books on my Kindle, including ones I had given up on in hardcopy. I feel this experience has significantly improved my Spanish.
The Kindle Paperwhite improves the experience I described above in two ways. First, you can just touch the word you are interested in rather than using the irritating joystick (called the 4-way). Second, the Kindle will fail over between different dictionaries. If a Spanish word doesn't appear in my Spanish-English dictionary, then the Kindle searches in other dictionaries, such as the Spanish-Spanish dictionary that now ships with the device. And if a word isn't found in any dictionary, the Kindle goes to the web -- though at this point I generally just go on with my reading.
By the way, someone with a PhD in linguistics gave me some advice about choosing books. The ideal reading level for learning is one at which you need to look up about one word per page. So try to avoid reading that requires more than that. Furthermore, if you understand the gist of a sentence, then have the confidence not to look up words in it even though you might not know them by heart. You will learn more by working on the fluency of your reading than by interrupting it to solidify the meaning of every last word.
