I would buy the newer 660 over the 550.
- higher res screen
- better battery life
- double the way points
- lane assist + 3D
- Bluetooth A2DP (stereo)
- "where am I" feature
- Querty or ABC keyboard layout
- Photo navigation (navigate to geotagged photos)
- World travel clock, currency & unit converter, calculator
- MSN® Direct compatible
- Speed limit indicator (displays speed limit for most major roads)
- Garmin Garage™ voices compatible (download custom voices to your device
The only thing the 550 has, which may or may not be too important, is XM® Navtraffic & Radio for U.S. compatible.
The 660 is $100 less at retail, is the newest model, and for the most part, is more feature rich. I personally think the 660 design is much nicer, have always thought the 550 was a pig of a unit. I know it's a proven, functional device, but it's FAT & ugly IMO.
Food for thought......