What exactly are you trying to do? If you really want to optimize your fuel management, get a programable system like Haltech. If you have a modified engine, it is especailly useful, as you can tailor the fuel and timing curves on the dyno. I have seen totally stock Mustangs pick up 20-30 hp at the wheels just by switching to an Accel DFI and tuning the system well. Also, as you continue to modify your car, you can adjust the system accordingly.
The other plug-in solutions are a bit cheaper, but they are Band-Aids based on averages and guesses. Just my 2 cents.