I just installed one of the iPod2Car adapters in my GMC truck. Installation was extraordinarily easy -- the kit comes with the correct plug to go into the back of the factory stereo CD changer input, a black box with dip switches to configure for the particular make and model of the vehicle, and an extremely long iPod 3G or 4G adapter cable that both provides audio output to the stereo and power to the iPod. The trick is to ensure that you provide power to the yellow cable that is left hanging out from the harness; the instructions say that this does not need to be done for my vehicle, but it does indeed need to be done for the unit to work.
Once installed, I found that I could shift through tracks using both my stereo and steering wheel controls. The only thing that's really missing is the ability to cycle through playlists and the ability to see the ID3 tags on the stereo display. Aside from those small shortcomings, the sound quality is amazing -- if you have the means to purchase one of the direct input kits, I highly reccommend it over a FM modulator.