I do notice a lot of people keep their killawatts plugged in all the time and this intrigues me. Why do this when you only need the measurement once of a single item ? I mean, if somethings running a 1000W or w/e I doubt it's kwh is going to fluctuate any if it's on 24/7.
I can tell by the power use if all my miners are actually mining. I can also tell, more or less, which one has stopped working by how much power isn't being drawn. It's a very quick way of telling if my miners are all mining without having to fire up the PC or laptop or whatever and check. If everything is running, I know it'll be drawing 800W. If it's only drawing 650W then I know the ASICMiners have stopped. If it's drawing 700W then I can almost be sure the Jalapenos have freaked, etc, etc.
I just use a generic Chinese meter, but it's good enough that it can tell me how much it's costing me to run my miners - I just put in the cost of a kWh and it shows cost on the screen.
Edit: This is the one I have:
http://www.amazon.com/Weanas-Energy-Voltage-Electricity-Monitor/dp/B00DTMQ1S6/ - only with UK plug, obviously.