Pros: small, inexpensive, accurate,
Cons: no backlighting
The main reason that Prius drivers get such good mileage is arguably not because they are such efficient cars, but that the real-time feedback of mileage makes drivers radically change their behavior. Enter the Kill-A-Watt. I bought one for my residential energy audit company last year and use it every time I go on site. I basically plug an appliance into the device, get a readout, and take a picture. This seals the number and appliance's energy usage into the customer's brain. From then on out they know exactly how much power it uses. It can really change the way people act in a very concrete way. Buy it for your own home for a similar revelation.