I have a Logitech keyboard on my Air, which isn't a case, it snaps closed over the screen, giving it good protection, it doesn't add much bulk or weight, it is very slim, and good typing experience.
I also have an Apple Bluetooth keyboard.
I use just a Smart Cover when I don't need a keyboard.
I also really like the Zagg Keys Pro, which I have on my ipad4. It is heavier, very solid, and has backlit keyboard.