Well, here is another point of view.
I have been looking for a replacement for my trusty ol' SP 2820, which may be getting tired, but - most of all - does not look current or cool. I am afraid to be met with derision from all of you sporting those slick thin GPS receivers.
So, last month, I bought a 660, and temporarily installed it on my GS for a daytrip. The functionality was about as advertised (I did connect audio via BT into my Sena intercom). To my surprise, the screen brightness was about as good as on the old-in-the-tooth 2820 - I expected much more, convinced that the 2820 was getting dim.
I liked the customizability (if there is such word) of 660 display information: a nice touch.
Now, there were some bad news, though. Few days later, I had to drop off my nephew at JFK Airport and drove there with both the 2820 and the 660 on the dashboard of my truck. The 660 hardware is certainly improved since the 2xxx StreetPilot days: receiver captures faster and recalculates a bit quicker.
But: the software is very bad for routing navigation. The weighing given to expected time over city streets is plainly incorrect (that means, the expected speed over city streets is much too high).
As I knew the particular route well, I knew which connections to take - and these were mostly highways. My 2820 concurred, always indicating my choice of route.
The 660, on multiple occasions in NJ and in NYC, routed over some convoluted path over back streets (and no, "avoid highways" was not set - I checked). Curiosity getting the better of me, at one instance I decided to follow the 660 to see if it "knew" better; that cost me an extra hour. For those who know the area: going from Verazzano Bridge to north-western NJ (correct route over NJ Tpke, then either I-78 or I-280), the 660 took me through city streets of Elizabeth and Union.
Now, I do know that a GPS receiver routing algorithm is not always perfect and local knowledge may yield a better route. But, the long list of Garmin receivers I had in the past, up to StreetPilot 2820 and up to Nuvi 265, seemed to generate proper routes for our area. The new - and improved? - algorithm in the 660 is not correct, a fatal flaw.
My use of the receiver includes not only leisurely meandering in map-only mode, but also navigating from point A to point B. In that respect the 660 failed miserably.
I sold it off the following week.
The 2820 will stay for a while longer, I think.
I may snag a 550 and try a comparison, though I expect that it may suffer from the same trouble, as both the 550 and 660 are based on the current crop of Nuvi receivers.
Just some food for your thought.