
The big advantage to the Cell Set is that (assuming you wear a headset on your helmet) that the audio from the Garmin goes through the intercom to the headset (and not the AUX input). All of the other audio inputs go to the headset(or speakers) as normal. The Cell Set mutes the other audio inputs for about 5 or 6 seconds while it sends through the headset gps instructions (or the phone or the Garmins mp3 player). You can set the Cellset to not mute the sound if you like.
You can run the audio from the Garmin to the Aux in but then the Garmin eliminates the XM, radio, CD/MP3 player. You could switch back and forth from the Aux to the radio but that is at best annoying (this is what I did until I purchased the Cell Set). Had over 1GB of songs on the Garmin but decided I wanted to listen to the other audio devices on the bike (epecially the XM).
I have had the system for over two years(three bikes) and have found it to be excellent. One more point, if you like you can have the HD sound (XM, radio and cd/mp3 player) routed to the speakers and the Garmin (and intercom and CB audio) info through the Heasdset intercom (as does HD when you wear a headset and listen to the speakers). This actually works much better than I would have thought especially if you have a half helmet with the new headsets.
The only hard part about the CellSet is you have to remove the fairing and wire the device into the HD audio system. Instructions are clear and everything but a heat gun is there. No soldering is required. Everything is compression connectors (so you can fairly easily remove the Cell Set and resinstall it on your next bike, like I have done). Works straight out of the box. Even my dealer has installed a few.
LIke I said earlier, if you have the XM on the bike, I think the Zumo 660 is better than the 550 due to the advanced Bluetooth setup, better screen and overall better unit (although not much).
The J&M headsets, however, are a great addition to the system. Much more efficent than the HD headsets that comes with the bike, less volume needed and better sound. IMHO having used both.