If you go through and do the calibration, the FE calculations are actually pretty darn close.
We do the math (odo miles vs gallons used) and the dash computer reads about 2mpg low per tank. After calibration, the Kiwi came within 0.6mpg of accurate (tested over a 3-tank run).
My wife also has the Torque app loaded, and sets the Kiwi to display "instant fuel economy" on her Android (full-screen, digital bar gauge) and props it up on the speedometer bezel. She's learned how to plot the data, and is kicking my butt now by 2mpg over her last 2 tanks.