Kill a watt, how accurate?

I have 2 of the P4400s and have found both to be surprisingly accurate after cross-checking them with a few other measuring devices including a revenue meter. Many light bulbs and heaters don't come very close to their advertised wattage, plus you have to know what voltage they're rated at to even know what to expect. As an example, while the standard rating voltage is 115, almost all hair dryers are rated at 125 volts. Because of this "nobody" will ever get the rated performance, but the higher wattage looks good in the advertising. The only places I've found the P400 lacking are the frequency and power factor readings with "modified sine wave" power. Even with MSW, the other functions give what looks to be good true RMS readings.