Install was a snap! Literally, you just take off the dash panel and unscrew the radio, and plug the harness in. I tucked the USA-Spec interface behind the dash, just below the radio. It's pretty small, and there was plenty of room. The aux-in and iPod cables run down and come out under the dash to the center console, where I have my Sirius receiver and iPod Nano plugged in.
Using it is just as easy - it's as easy as pressing your CD player controls! And the sound quality is 10x better than the FM transmitter stuff I was using - that alone was worth the cost of this little box. It's kinda nice being able to change tracks on the iPod with the deck controls as well, since the Nano is so small.
Overall I give the USA-Spec two thumbs up, as well as Crutchfield where I ordered it. They include full instructions on dash panel removal with the kit (specific to your vehicle), and as always I got what I ordered from them quickly and without any trouble. I highly recommend it!