OK, I've had a little road time with this so here's the skinny. This thing is an interface that allows you to break the link between your factory radio and the XM tuner, and put a module in that gives you a HDradio tuner and an iPOD controller.
First for the HD radio part. This thing works by emulating the XM band on your factory radio, so the display shows all of your AM/FM stations with an XM number. At first this can be kind of confusing, but once you realize what's going on, you'll remember what the numbers are. For instance, I listen to 94.7 FM. The radio displays this as XM225. I now have it saved in the preset buttons as XM225. The confusing part is that I have to remember what this preset is for. If I press the info button it will cycle the station name, song title, and artist. On HD radio some stations have multicasting. For instance, 94.7 around here is KSHE. They have KSHE-HD1, KSHE-HD2, and KSHE-HD3. The first one is the same as the analog station, but the other two have different content, and are comercial free. The only quirk about this is that all 3 are under the same XM channel number, and can not be stored separately in the presets. I can save the first one then I have to seek up to the others.
The sound is excellent. When tuning in an FM station, it plays the analog signal for a few secconds until it locks in the digital signal. When it switches over, the difference is amazing. The sound is crisper and fuller, as well as louder. I don't know if the increased volume is from a higher signal or what, but it is also key to one of the downsides of the sound. The digital signal will ocasionally cut out, and the sound level drops significantly. If it cuts out while listening to a multicast station, it goes back to the analog station. This signal dropping gets worse as you get further from the source. The range of the digital signal is not as good as the analog signal. there is no fuzzy fade of the signal. It's either there or it's not. It's just the nature of a digital signal.
Now for the iPOD control. First I must qualify that I am as new to the iPOD as I am to this interface. I bought my iPOD about a week prior to getting the interface. If you are familiar with the iPOD you know how intuitive it is. The interface is fairly easy to use as well, but there is a bit of a learning curve. As with the HD radio, this is also emulating XM, so the buttons used to controll the advanced functions of the iPOD are assigned an XM number and need to be saved in the presets and remembered what they do. For instance button 4 is saved as XM7 and this allows you to enter browse mode, and button 5 is saved as XM8 and cycles through the shuffle options. The buttons are not labled for what they do. they merely have the corresponding XM number above them. This is annoying, but once you remember what each button does, it works pretty good. The communication seems kind of slow between the iPOD and the radio, but from what I have read the PAL unit you can get from GM works very similar. The sound is also excellent.
I did also get an aux input cable that I can use instead of one of the two input sources, but I have not used it yet. I asked about a sirius tuner, but they said none is available for this application yet.
So far, I would have to say that I'm not sure if this was money well spent yet. The sound is great, and the ability to have these two inputs available is nice, but I wish it was more user friendly. This is probably the only way right now to add these two inputs to the factory radio. I am sure that in the future there will be factory radios equipped with these features, but it might be a while. There are probably aftermarket units that offer these features, but I did not want to replace the factory radio, so this was the way to get them.