I have the same exact model of voltmeter (well, nearly the same--mine uses blue LEDs for the display, because that was a buck cheaper at the time) waiting to be installed. When I compared mine with my Fluke, the two agreed quite closely, with about a .03 V difference as I recall. Depending on where you're measuring the voltage at in the system, you could very easily get a larger difference and variation just from wiring losses, particularly if there's some nontrivial load on the particular 12V circuit. I'll be running a separate (properly fused) 12V sense line and a dedicated ground for the meter to avoid this.
(Obviously, with these cheap meters, the calibration may not be consistent between units. The advice to check individually is good.)
