You can also use standard AA alkaline batteries if you're in a pinch, but the Micro-USB cable and port on the top of the keyboard make it easy to recharge using a simple computer connection, and Logitech rates the battery life for up to 10 days on a single charge.
Like many of Logitech's peripherals, the K800 is part of the Unifying receiver family that lets you connect a single USB plug to six compatible mice and keyboard devices. Each one comes with its own Unifying receiver, meaning that you can conceivably leave them all plugged into several different computers at home and at work. Keep in mind that buying into the Unifying connection means locking yourself into Logitech products only, but we can't ding Logitech since the Unifying design also frees up USB ports previously occupied by other devices.
Our only critique of the K800 is the fragility of the keyboard's chassis. We've tested plenty of Logitech keyboards in the past that felt solid and unbreakable, but the K800 allows for a significant amount of flex, so much so that we're wary of its long-term durability. We assume that the wafer-thin profile has a lot to do with it, but the amount of bend allowed by the keyboard is suspect nonetheless.