Former Amazon engineer Andrew Munn:

From the moment you pick up the strangely weighty slate and press the tiny and awkwardly placed ON button, the user experience is abysmal. The swipe-to-unlock gesture is laggy. Let me repeat that, the swipe-to-unlock gesture is laggy. This is the first interaction a user has with the device and Amazon couldn’t even get it right. Adding insult to injury, it’s an extremely simple and ugly unlock-gesture, just the sweep of a mono-color bar from right to left.