The Cobb Access Port is a handheld device that lets you back up your stock map, then reflash the PCM to 5 different settings, each one according to what you want out of the PCM and your modifications. It does things like optimizes the air/fuel ratio, ignition timing, displays live vehicle data (coolant temperature, oil temperature, throttle position, intake cam advance, Air/Fuel voltage and many more), and it also has a troubleshooting mode to read OBDII codes and clear them. It has a valet mode for RPM not to exceed over 3200 RPM and an anti-theft mode.
It just plugs into the OBDII port and is super easy to use. You can search the forums for what people say but it's suppose to give a 5-8% boost for HP on a stock car. I got it because I have intake, headers, and full exhaust. I usually ran the stage 2 plus and could tell a noticeable difference. I think I paid $600-$700 for it new from Cobb. Once it's installed on one car, it's paired with the car until it's uninstalled, so I took a picture to show it is uninstalled and available for tune! Any more questions, just ask. Thanks