Anyone use these gadgets? There's one on dealextreme http://www.dealextreme.com/details.dx/sku.35152 that has clock/IN/OUT temp/voltage. I'm more interested in the thermometer but figured the voltage meter was a bonus. I also like the display lights.

How do these things work. Do you leave it plugged in 24/7? How accuarte are they in reading your battery power?