I have had a Kindle Keyboard 3G for four days now and have read a couple of books and long essays off of it.
The device has a good shape and size, and the screen is pleasant to read from. It doesn’t work terribly well with unconverted PDF files, which is quite a pain since the main reason I got it was to read thesis sources with. That being said, you can use Amazon’s free PDF conversion service for files under 50 megabytes. The converted files get delivered to your Kindle via WiFi. Unconverted PDF files load very slowly and clunkily, and sometimes cause the device to freeze up. All told, the interface of the device tends to be frustratingly slow. Even highlighting a passage of plain text can be a patience-trying task. Often, interacting with the Kindle consists of pressing a button and then waiting 5-15 seconds for it to have an effect.
The built-in web browser is poor, but good enough to let you use the free WiFi at Starbucks by clicking the button to accept the terms and conditions. One nice connectivity feature is that you can select a passage, write a short comment on it, and post the whole thing to Twitter. This is available by WiFi only, not over the 3G connection. I also like how the Kindle automatically collects all the passages you highlight in all documents into a single ‘clippings’ file.
The keyboard is tolerable for writing short notes, but you certainly wouldn’t want to write an essay or email on it.
All told, the Kindle is a pleasant and effective way to read plain text files and other properly-formatted documents. It isn’t great as a PDF reader, though perhaps future versions will be better in that way. One thing to be aware of is that – in my experience – the claimed battery life of the Kindle is a vast distortion. Amazon says that it will be good for 1-2 months, based on 30 minutes of reading per day and no wireless connectivity. I have found that I use about 1/3 of the battery every day. Admittedly, I have been using it for a lot more than half an hour. Still, my own use suggests that the battery lasts for about 10-15 hours with wireless turned off, which is better than a laptop or iPad but not sufficient to let you travel without worrying about finding places to charge.