I use a bluetooth one oftentimes to monitor "trip mpg". Don't know how accurate it is though. I bought this from ebay for about $15, and also bought Torque app for Android.
I also like watching the exact throttle percentage and fuel level in decimal number.
RPM is also something I look at and it's not consistent, maybe because the hill assist is being applied or sometimes the AC is on but it's actually alternating on and off.