I checked mine versus the computer a lot during the first year and it seems pretty accurate. I found it is more accurate when you go a steadier speed and less accurate with lots of acceleration and stopping. That makes sense because the computer is working with an average speed and distance stats along with the fuel usage (without accounting for evaporation or any other loss of fuel).

Mine was always within about 1 mpg or so. It's valuable for monitoring your general engine condition - you notice a sudden drop in mpg without changing driving patterns and you can investigate a possible problem.

Also I found it most accurate on road trips, when you need it the most!