Just a quick update for those wanting a side-by-side comparison between the front runners in this category....I bought the USA Spec adapter a few months ago with the intention of hooking up a bluetooth transmitter and getting a fully wireless solution. That didn't quite work out as I was expecting, so I just picked up the GROM unit with the bluetooth add-on. Based on some limited experience with each unit, I would probably recommend the GROM unit simply for flexibility.
USA Spec:
I can't comment too much on the playlist functionality since I tend to use my ipod in hybrid mode so I can listen to streaming radio (pandora, soundcloud, etc.). One of the things that I didn't care for on the unit was how plugging in my iphone would always launch into a playlist mode and put the phone into accessory mode. This became even more annoying since it would start playing random music from my itunes library that I didn't care to be listening to. Otherwise, sound quality was great, and I have no complaints about the unit.
More recently, I moved this unit into my daily driver, a 2001 Acura TL.
GROM Audio w/ bluetooth:
I bought the ipod kit with the bluetooth hands-free add-on. First, a comparison between the units. The ipod functionality on the USA Spec seems to be a bit better. Again, I don't use the playlist features, and run this unit in hybrid mode, but I've found the integration to the HU to to be less intuitive. I'd have to test again, but it seems like all the HU buttons don't map perfectly. The sound quality appeared to be on par, and I was happy with that aspect of the unit.
The bluetooth capability of this unit was definitely the selling point and so far it hasn't disappointed. One issue I still have (that I have with hybrid mode as well) is that playback doesn't automatically start or stop when entering/leaving the car. This means that I can't leave the phone in my pocket completely, which was my real desire. But having hands-free in the car is definitely nice, and as I use my iphone for navigation, having the music mute for prompts is also nice. I haven't wired in the mic yet so I can't comment on the hands-free quality yet, but I can definitely comment on the support for GROM. In my initial testing with the unit, I've had some issues with a constant ticking noise coming from the speakers when connected over bluetooth; GROM's been extremely responsive in trying to sort this issue out and they are working on a new hardware revision to have it fixed. Note, since it's still sitting in my trunk, I just disconnected the ipod cable from the unit and that removes the ticking in the interim.
Conclusion:
Both units are actually pretty good, but neither are a perfect solution. The price of the GROM unit with the bluetooth add-on is just slightly over that of the USA Spec unit, which makes it worth purchasing over the USA Spec, in my opinion.
If anyone has any specific questions about either unit, feel free to ask.