Yes...it's great for tracking instantaneous MPG, current trip MPG, tank MPG, miles to empty, etc. Highly recommended if your car's doesn't already have a built-in computer capable of displaying such information. The nice thing is that it can be popped out of one car, and placed easily into another. Plus, it can scan OBD-II codes as well.
I've been thinking a lot about getting one, but haven't shelled out yet.
It's important to check the accuracy of the scangauge or trip computer against real-world mileage. Sometimes they're very accurate and sometimes they're 10% or more off all the time. I recall seeing one fuelly user who logged their tanks off gas receipts and then put the mileage that the trip computer said the car got over that tank in the notes section. (It was a late model Honda Fit or maybe a Civic.) The trip computer always read about 10% higher than the gas receipt/odometer calculation did. Wish I could remember who that was.
My Grand Caravan has one built in, with live MPG right on the overhead readout, along with avg MPG, time, etc.. Its nice, but annoying. Most of the time on level grade at 55 I get well over 30MPG. LOL But the results tell another story. The avg MPG gauge is usually close to what the site shows. But it gets reset every oil change unlike here. Winter its worse, Summer its better. Ah well.
I have one and it is great, You have to calibrate it with a GPS and it is dead on now with that. My MPH and odeometer on the car says I am going faster that I am. It can be gimicky but I use it for all my fill ups to give better accuracy. I also fillup to top of the tank filler neck to get even more accuracy.
Keen4Green - I have one for my 2010 Insight, I have found the MPG reading to be less consistant than the onboard MID (which is consistantly optomistic). I would still recommended it. It is useful for water temp (a must if you are considering winter grille blocks). It is also useful for showing fuel cut mode when the MID display is on MPG (the Scan Gauge reads 9999). And it can give you a trip MPG reading if you are using A trip meter for tank and B meter for life average.
For diesels, the ScanGauge is great. Very versatile. On my SG, I've set up a turbocharger boost register plus a real-time horsepower register. You can display up four registers. On mine I have real time turbo boost in PSI, real time MPG, tank MPG and current trip MPG. And they are great for gas burners as well. Above all, what it teaches you to do is use the least amount of energy to get from point A to point B.