I have a 450 and it works great for my needs - I didn't need the Bluetooth.
I know lots of guys that have used the 550 and others with the 660. I hear few complaints. Garmin is probably the most widely-used motorcycle GPS.
To me, the GPS is one of the greatest tools you can have on your bike. If you learn to use the computer-based mapping software you can create some fantastic routes and help you avoid interstates and other boring roads. In the last 4 years we've ridden tens of thousands of miles with the Garmin on board.
Luceo Non Uro