I never owned a 550, but I bought the 660 because I already have XM radio on my CVO Ultras and have put about 45,000 miles of riding on the 660 since the unit was released by Garmin in May 2009. I am very happy with it. It takes a while to get to know all the features, such as hotels nearest the current location vs hotels at the destination. I also bought the lifetime unlimited map updates from Garmin for $100. Not only does the unlimited map deal update the map on your 660, but also in Mapsource on your computer, so you can upload routes, waypoints, custom POIs (all the HD dealers in the US & Canada), etc using the same source map and target map. The custom POIs come in handy when you need an oil change, I think garmin updates the map about 4 times per year.
I put a 16 Gig memory card in it and never have taken the card out. I load all my music, playlists, etc., into the 660 by plugging the usb cable from my computer directly into the garmin with the battery cover off. I have about 50 playlists with 600 plus songs and only use about 3 gigs. It works slick. I also bought the phone interface from James Bissonette so when I bluetooth my blackberry to the garmin, I can talk on the telephone through my J&M headset mic when the garmin is in aux mode (which is the same mode as the MP3 music, so it's easy to know when you get a call because the phone rings and interrupts the music). That feature works relatively well, but there is room for technology improvement with the mic quality (people being able to hear you clearly). I have that set up on two Ultras and people on the other end of the phone can hear me okay. It's definitely not as good as a bluetooth earpiece I wear when not riding but it's better than not having anything.