I got one of these http://www.amazon.com/Soliport-Bluet...1916533&sr=8-1
and downloaded Torque off of the Android App market.
So far its pretty freakin' sweet. I've been running it for a week to see how accurate it is and how well it adds up gas usage. It has features like current mpg, throttle position, pitch/roll meters, gps data, logs where you went and how you drove to figure out driving habits, and can read about every sensor that you available in your truck/car etc..
So far for MPG its only off by about 1 gallon from my math, vs the software's read outs. My math is spotty so it could be right on, either way close enough. I ran 2 tanks from topped off to almost having to push it into the gas stations, and so far 1 mile off each time which isn't bad.
I took my truck to my mechanic who has hooked up my truck to his machine, took readings, then we tested my obd2 scanner to see what my 20$ scanner is getting vs. his $$$$$ machine. They are very close, most of the time the OBD2 adapter is normally less than what it's really reading but not by a lot. Depends, Air/fuel was spot on with his systems, but Cat/O2 temp was off by about 5-10 degrees. It got better as the truck warmed up more. I've heard that better quality bluetooth scanners do not have any issues with readings.
So if your a gadget freak like me, the paid version of this app is totally worth it. It has a lot of features, very easy to customize. I got the scanner for 22$ and the app was 5$. Loving every minute of it, still a lot more to figure out with it too.