I bought a 550 from someone who upgraded to 665... he said he liked the 550 more... but from my point of view:

550 is more snappy

The 66x range doesn't have the buttons on the side... i like those for zooming, changing volume and such.

Battery life on 66x range is lower, i do get it of the bike for a hike when going on foot...

regarding 220 range... it does lack a lot of the features i wanted - wired audio in / out, mp3 player, buttons to avoid using touchscreen with gloves, bluetooth communication with a phone.

660 is more of a touring bike gps than 550 / 220 - easy to fit on a K / RT / GTR / etc but not so easy to mount on the handlebar or under the screen on Tenere.

Regarding other options ( TomTom etc ) - they are really not useable out of Western Europe due to lack of maps... you can get very good maps for Garmin, including topo / offroad and such but not for TomTom.