I've installed head units in our daughter's 2006 coupe and recently her 2007 sedan. I went with the basic wiring harness for the 2006 because the chime and blinker sounds aren't built in to the stereo in that model year, but I did have to source an illumination and an ignition wire to make everything work. It wasn't very difficult but a bit more than your standard harness wiring.
For the 2007 I went with the plug-in adapter because it retains the chime and blinker sounds, and also supplies the illumination and ignition wires (I believe it may even do retained accessory power). It was a little pricier than the basic harness, but well worth the money (plus Crutchfield highly discounts the installation gear). I strongly recommend just picking up the adapter, and the installation is then literally a direct one-to-one of connecting the radio harness to the adapter.
The adapter I used was "Metra LC-GMRC-LAN-03 Wiring Interface"
