They're pretty accurate. It's the same info your PCM is reading from your sensors. One benefit is that it gives you a better idea what your trans temp and engine temp is as a quantitative value instead of the dummy gauge on the dash.

Here is a pic of mine with trans temp (TFT), engine temp (FwT), volts, and RPM. You can display 4 gauges at a time. It plugs into your OBDII port constantly. It can also read CEL codes. Just disconnect it when going for your smog check.