My friend Ryan and I, mainly Ryan, installed my Alpine KCA-420i iPod Interface last night and I have to say it's pretty nice for a $100 unit.
The interface allows you to control your iPod from any AI-Net enabled Alpine head unit.
The install is pretty damn easy as all you have to do is plug a cable into your AI-Net out to the 420i and then plug another cable from the 420i to the iPod. Alpine gives you six feet for each cable which was enough to go from my head unit, behind the glove compartment, down the right passanger side trim and then under the seat. The other cable goes from under the seat to the glove compartment where the iPod is stored.
The ability to have 20 gigs of MP3s as apposed to 700MB is quite nice but the interface takes a little getting use to. From reading the manual it looks like people with newer head units will have a more intuitive interface but I can't complain since my 9815 isn't technically support. If you have a 9815 you just need to follow the instructions for the 9835 model.