My opinion on the K2 can be summed up simply. If you are getting it as your first Kindle, you will probably be happy (except for the joystick). If you already have a Kindle, then I see no reason why you should upgrade. The K2 is only an incremental improvement on the original.
The K2 does have several nice features. I really like the TTS. I've used MSReader for a number of years; the voice it uses is at best grating. The K2 has a significant improved voice with both genders and 3 speeds. Even though it is nice, the people who could benefit most probably cannot use it. It's possible for a blind person to activate the TTS by memorized keystrokes. However, I do not find it feasible that a visually impaired person could navigate the menu and select a specific book. This reduces the TTS to a fringe benefit, instead of a main feature.
I also like that they fixed the keyboard. The original keyboard was apparently designed with a Blackberry user in mind; it was really a thumb board. The page turn and other buttons have also been changed; they depress on the inside edge. This will significantly reduce the annoying accidental page turns.
The screen refresh and battery life are both somewhat improved over the original. I was quoted two weeks battery life, and 20% faster page turns. True, they did effectively double the battery life, but will you benefit from it? I would not gain anything. I have a pattern of charging my Kindle every Sunday night. I will still charge every Sunday, just to be safe.
The K2 does have some shortcomings. In spite of what Amazon has said, I still think it was a mistake to drop the SD card. I liked being able to remove the card and insert it into the card reader in my laptop. I don't always have the correct cable with me, so the SD card was a guarantee that I could still add content at any times. And no, I don't want to send it over Whispernet; I don't always have a connection.
That brings me to the USB cable, which I regard as a blunder. Using the same connection for power and data isn't the problem. The problem is that Amazon dropped the USB mini-B plug in favor of what I'm told is a micro-USB plug (thanks tompe). I'm told this will be the new connection for cell phones. Even so, I regard this plug as funky bizarre non standard plug because none of my other devices use one. If I got a K2, I would have to carry around this cable to be used on exactly one device, and a second cable for my other devices.
I also have serious doubts about the joystick. I did not find it easy to use. But I really only used it for about an hour. I look forward to hearing the opinion of the first K2 buyers.
One final note: The battery is not replaceable. This hasn't bothered me on most devices, and I suspect that you would never need to replace the one on the K2. But I do agree with the those who at least want the _option_ of replacing the battery. I do think that having the option available is important. This is a point you should consider before buying the device.
P.S. I very specifically avoided commenting on the changes in the design of the case between the original Kindle and the K2. There is a quagmire of fugly original versus Applish K2 arguing that I refuse to be dragged into.
P.P.S. I left out a number of things like folders, as well as other ways that the two devices are identical. If both devices possess or lack a feature, then I saw no need to mention it.