Pros: Easy, informative
Cons: Might have more features than most people want
A friend loaned me one of these for a few months and it really helped me understand power usage around the house. It's pretty simple. You plug it into the wall, and then plug your electrical appliance into it. The digital readout tells you how many watts the appliance is using. It's that easy. It has a few options, too, that can provide more information.
For example, you can leave it on all night and it will tell you how much cumulative power the appliance used overnight. This is helpful for things like refrigerators, where the power usage goes up and down as the internal mechanisms switch on and off again. It helps to learn a couple of basic concepts like what a kilowatt-hour is, and amps versus watts. But even if you don't understand all that stuff, you can start comparing different appliances around your house to each other, and find the big power suckers.
For example, we had this great hot-water-kettle we loved to use. Basically, it's a sealed thermos with a pump spout. You plug it in, and then just leave it on. It keeps water piping hot all the time. You never have to wait for the kettle to boil, just go up and get hot water when you want it. It made morning coffee much faster! HOWEVER, using the Kill A Watt, we learned that this thing eats up 80 watts of power continuously to keep the water warm.
Would you leave an 80-watt lightbulb on 24/7 just so you could make coffee 5 minutes faster? We unplugged the thing. Other interesting things we learned were how much power appliances use when turned off. It turns out our TV uses almost 5 watts just sitting there, listening for the remote control to turn it back on. Many other electronics devices around the house do the same thing, and we've started unplugging the ones we don't use often, sometimes re-routing the cords to make this easy to do.
We also learned that our electrical radiators use an absolutely astounding 1500 watts when they're turned up to high. We thought we were smart for not running the central heating, but it turns out that our central heating might be more efficient than two of those space heaters.
And we invested in a 4 watt electric heating pad for our cat to sleep on so we wouldn't have to run a 1500 watt space heater in her room overnight. Once you have the numbers, a lot of these decisions become obvious. Kill A Watt gives you those numbers.
I definitely recommend this for anyone who cares about saving money and energy. Learn the basic functions and start testing around the house!