I have two SGII's. One in my Pathfinder, and one in my Mazdaspeed6.

In the Pathy, I use it to display voltage (VLT), engine coolant temp (°FWT), instantaneous MPG (MPG), and either calculated engine load (LOD), manfold absolute pressure (MAP), or throttle position (TPS), depending on conditions. It's also useful for displaying accurate MPH if you have oversized tires.

In my Mazda, I use it to display MPG, MAP, ignition timing (IGN), and LOD. Since my Mazda is turbocharged, the MAP display can show boost pressure in absolute PSI (i.e. boost = MAP - 14.7psi)

The SGII calculates your average MPG during the current drive cycle, the previous day's MPG, today's MPG (if you have multiple short errands through the day), as well as overall MPG for the current tank of gas.

I find it very useful to track in real time the effects of driving at different speeds in my daily driver (Mazda). Yesterday, I drove 80mph on my freeway commute, and got 22.3mpg. Today, I drove 65mph on the same route and got 26.2mpg.