I still own the 550 myself, but went to check out the 660 in the local Garmin shop and know some people that have bought it.
The larger screen on the 660 is definitely a big plus, lane choice indication is nice as well, although not that big of an improvement.
I think I would miss the buttons from the 550 though as I frequently use them to zoom, change the volume and flip through the map view, fuel view and music player view.

The 660 also still has some software issues and annoyancies. Garmin have a tendency of releasing a new product before the software itself is entirely finished. They do provide regular software updates though so that problem should solve itself.

If I had to buy a new GPS device right now, with the current special offers the 660 would probably be my first choice.