I got a couple of these devices, one for work, and one for home.
At work, it was reading 1.3kW. I have a plug-in power meter (which measures amps AND volts, unlike the Efergy which just measures amps) in our server room which was measuring 670W or 780VA. This feeds into a UPS which was reading a load of 3A (so about 750VA - similar to the plugin-in meter). When I unplugged the UPS supply, the Efergy dropped to about 1kW - so it only measured a drop of 300W, not the 600-700W it should have done. That's a big difference, relatively speaking.
FWIW - our annual electricity bill suggests we use an average of about 2kW all the time (which is why we are seeing if we can monitor it better), so during the daytime, it looks like the Efergy was reading quite low - maybe the 1kW was right, and the 1.3kW was wrong - you can't easily tell.
Then I tried the other Efergy which was destined for home, which consistently gave readings 200 watts below the first one!
Just to add even more fun, we had great confusion because when we turned OFF some fluorescent lights (about 80W worth), the reading went UP (by 300W, not an insignificant amount!), and when we turned them ON, the reading went DOWN - so according to that we should leave the lights on all the time! (As far as I can think, this may be due to capacitive and inductive loads cancelling out when the lights are on - however it makes the devices hard to use for what they're intended for).
So, while these things are easy to install, and can give a rough idea of power usage, don't take them too seriously, and be especially careful if you have fluorescent lights or lots of computers.
We may need to look around for a smart meter which can take power factors into consideration, even though it will need an electrician to wire it in.