Short answer: very very accurate.

Long answer (read at your own risk, bring coffee):

I have 27K miles of manual logs so far that I keep in Excel to analyze the car. I measure the mileage manually each tank and I also record what the trip computer metrics are for each tank so I can run comparisons. And when I say metrics I mean ALL of them: trip odometer miles, actual odometer miles, mpg for that tank as recorded by the car, DTE when fueling, avg speed for that tank, whether or not the fuel light came on and if so what mileage was the trip odometer at when it did come on, what was the DTE reading when the fuel light came on, and if I let the car go to 0 miles on the DTE readout at what mileage on the trip odometer did it occur, gallons used, price paid and price per gallon.

I can tell you that based on my model ('15 2.5sGT hatch, manual trans), with my driving style/commute, etc., I have never had any individual tank have an error of more than 0.9 mpg in either direction. And most times it's fairly close to accurate. In fact if there's a wide deviation on a particular tank, I'll often notice the next tank tends to correct the other way. That indicates the fuel pumps at the stations themselves aren't precise in their cutoff methodology (despite my very precise way of fueling my car -- fill it, when it cuts off, count to 60, then top it off 'til one more click) and generally supports the accuracy of the trip computer.

Over the life of the car so far my real world mileage -- total miles driven since delivery divided by total gallons used since delivery -- is actually only 0.2 mpg higher of the calculated mileage by the car's trip computer (and I do it the precise long form way -- that I won't get into here -- not just the average of the car's tanks recorded each time).

I'm averaging about 35 mpg lifetime (probably a little less since we're in winter and it falls in winter) so that's a margin of error of about 0.6% which is pretty solid.

So yeah, it's pretty accurate based on a sample size of me.

You should see all of the other metrics I run on this thing.