It's interesting to find this thread. I bought and installed USA-SPEC when they first came out with the BMW iPod connector, and it was so new I actually had to pre-order from them... 4yrs ago or so. I had decided I need more than just the in-dash CD, and figured out buying a new iPod and USA-Spec interface would be much cheaper than buying a BMW CD changer! and with the iPod, you have less of a limitation of "70min music per CD" stuff.
I ran the cable through the folded seats, wires tucked along the edge and the floor (below floor mats, above carpet.. I'm not a perfectionist). the USA-Spec module is sitting on the floor of the front passenger seat, and the cable just tucked between the parking brake console and the passenger seat. I didn't bother routing it nicely. sure it has some extra cable dangling, but hasn't bothered me.
anyhow, I've had this for years, and LOVE it. only problem is that with my old iPod with much poorer battery technology, if I don't drive my car for 2 days (meaning, if I stay home all weekend) my iPod won't go to sleep when connected to the USA-Spec cable, and thus end up with completely discharged battery... and if the iPod is dead and won't even boot up, the USA-Spec won't be able to charge the iPod.. so I had to buy an iPod car charger, just to have teh ability to give it enough charge to boot up for the USA-Spec to recognize the iPod and start charging it. But this is more of my iPod problem and probably not much of an issue with the newer iPods with much better battery technology. If no one else has this problem with their recent purchase of the USA-Spec, then may be they improved something to allow the iPod to go to sleep when powered off. Dunno.
other than that, it's been working perfectly.. I don't have any noise issues or interference issues with it either. oh... I did have to "reset" the USA-Spec once... but that was because I didn't drive the car for weeks with a dead iPod connected, not to mention the battery on my BMW also died during that time (long story). Simple fix though--just had to unplug the module from the power and then reconnect it. otherwise trouble free...
Oh, my mother has an iPod Touch 2nd G. and that works perfectly with the old USA-Spec too...