I have one of these, which is probably a similar device and price to the kill-a-watt.
I haven't opened it up to establish how it works , but it gives convincing readings which do bear some relationship to my meter rev counter readings. (I reckon my freezer uses about 100 watts and my new mini server about 10 watts both by the meter revs and by the power monitor ) I get high and low power factor readings when I expect them. Freezer 0.96, Computer 0.46.
I could probably get some better figures from my CT if I applied my mind to the problem, it gives terrible readings at low power usages.
I notice my site gets a mention on Jarv's page now. I really must do a write up........