tl;dr: A great (and very inexpensive) alternative to an Apple-branded keyboard. You get what you pay for, and a little more.
Ever since Apple introduced its first 'chicklet-style' keyboards on laptops, I've been a fan. I love the feel of a loudly-clacking Apple Extended Keyboard II, but I can type much faster and more accurately on Apple's newer keyboards, like the $69 Apple Bluetooth Keyboard. I also like the compact footprint and durability of the aluminum-bodied keyboards, even with the larger Apple USB Keyboard with Numeric Keypad ($49).
Unfortunately, though, Apple's keyboards are relatively expensive, and I've found that between my iPad, my Raspberry Pi, my desk workstation and my standing desk (and elsewhere), I always seem to have one too few keyboards. Lucky for me, I found a really nice $19.99 keyboard that feels close to Apple's quality, sits in the same footprint, and comes in white or black: the Anker Ultra Slim Mini Bluetooth 3.0 Wireless Keyboard (made for Mac, iPad, Android, etc.). The name is quite a mouthful, but the keyboard is a pretty good deal!
Hardware / Build Quality
The entire keyboard is plastic. But other than that, and the letters not being the exact same typeface and font as on Apple's own keyboards, this little keyboard is the spitting image of an Apple Bluetooth Keyboard. It uses 2 AAA batteries, which will not last as long equivalent AA batteries, and the battery cover flap feels a little, shall we say, 'breakable.'
The key action feels close to Apple's own, but you can tell it's not exactly the same. I'd put the key action somewhere between a modern Lenovo laptop and Apple's own laptop keyboards. Every now and then I feel I have to push down a bit harder than I would with my Apple keyboards, but on the whole, typing feels good.
The on/off switch (on the bottom, near the battery door) is the worst part of this whole keyboard; it requires a good fingernail-push to switch on or off. The Bluetooth pairing button is easy enough to press, for what it's worth.
The hardware and feel is much better than I was expecting for $20, but it's definitely not in the same league as the aluminum-bodied Apple keyboards.
Looks
I bought the black version of the keyboard because I like a more svelte look. I have three white-on-aluminum Apple chicklet keyboards, and I love the feel, but wish they were slightly less ostentatious. This keyboard looks great so far, but I can tell the paint for the key lettering will start wearing off way earlier than on my Apple keyboards.
Everything's plastic, so the keyboard doesn't have a premium finish (the key action is also slightly louder than on an Apple keyboard), but it still looks and sounds a lot nicer than any cheap Dell, Kensington, HP, etc. keyboard I've used. The keys are already getting a little shiny in places after a couple weeks of hard use, but I'm fine with that.
Bluetooth
The range is fine for my needs. It works within the 5' radius around my computer and tablet. The batteries (2xAAA) have lasted two weeks so far, but I'm guessing they won't last nearly as long as my nicer Apple Bluetooth Keyboard's larger AA batteries. Pairing was quick and painless. No complaints here.
Usage Tips
If you're using the keyboard with a Mac, you might notice you have to use fn+Esc to hit 'Escape', since the key is mapped to a non-existant 'home' key (for iOS). To make using Esc easier, install PCKeyboardHack and check the 'Change Caps Lock' option, then set it to keycode 53. Now your useless Caps Lock key is an Esc key (old trick I learned when I started using vim and didn't want to get RSI).
Recommendation
This is an excellent deal for a simple, very nice looking, secondary wireless keyboard (though I would recommend either the full-size Apple USB keyboard or an Apple Bluetooth Keyboard for your desk. I might buy another to use with my Apple TV and in the living room (so I don't have to carry a keyboard from room to room). We'll see! I give the keyboard five stars for value, four for build quality and usability.
Buy the Anker Ultra Slim Bluetooth Keyboard on Amazon for $20.