I put one on every plane. Well... I don't put them on the 6-volt planes. They're great $11 insurance. The plane in my Avitar has one concealed beneath the white Ultracote. The LED's shine through it nicely.

Losing a plane to a low battery is just Nonsense! Nonsense! I concur on the accuracy. I've tested 3 comparing to a digital meter - they're very accurate.

A suggestion: Don't just turn on the receiver and glance at the Voltwatch. Watch it while you're throwing the servos around. I don't fly if it dips into the yellow zone.