I have used the CellSet product (purchased form Cycle Gadget's ) in my last two CVO ultra's. By far the most functional and best add-on to my bike.
Allows the full use of the entire HD system (ie, FM,AM, Mp3, Xm, weather etc) and allows the Garmin prompts to be routed through the "wired" headset. Because the CellSet uses the intercom/CB channel of the HD unit it allows the Garmin to work fully intergrated.
Because the Garmin comes with the Bluetooth capability, the Cell Set will allow you to answer the phone and makes calls (if you want). i only use the home dial feature. And you see who is calling and thus can either answer or wait till you stop. The audio quality is so good most people don't even know you are on the bike.
Again, it it is with out a doubt the most used add-on I have purchased.