Whether you're paranoid about security or not, a motion-activated light at the back door makes good sense if only to make it easier to stick a key in a keyhole in the dark. That's the main reason why we bought a Cooper Lighting MS188W Motion-Activated Security Light.
I mounted ours on the soffit, up under the eaves, but you can also mount it flat on the wall. The light compartment is in the middle of the mounting arm, with the motion detector at the end. Installing it was fairly easy: it fits on an ordinary junction box and the mounting hardware includes screws to attach it to the box, a rubber weatherproofing gasket (be smart - use caulk, too) and wire nuts. It might have been easier to install if there were a built-in hanging strap for while you're connecting the wires, but that's what bungee cords are for, right? The hardest part of putting everything together was opening the light compartment to insert the included 300-watt halogen bulb.
After installing the light I had to program it, which was also fairly easy using the included instructions. The light can be set to stay on for 1, 4, or 12 minutes; and the motion detector has a sensitivity control, too. The detector nominally covers a 180-degree area, though it's most sensitive in the middle, and - depending on the height of the installation - has a maximum range of 20 meters, about 70 feet. A built-in photocell keeps the light from coming on during daylight.
The light worked just fine but only for about two years. When the motion detector unit just quit working, I had to throw it away and replace it - with a different brand. Although it wasn't hard to install and worked well while it worked, the lack of durability means I can't recommend this product.