I just updated from a Streetpilot c340 to the nuvi660. I guess I'm going to like it. At this point it has an annoying boot-up where I have to enter the locale, language, and time zone each time. The c340 never did this, so if there's somewhere on the nuvi to make it permanent, I haven't found it yet.
As far as the windshield suction, your windshield has to be cleaner than clean, and you might use a little Windex on the mount, too. My windshield is smoke-dirty, so I use a bean-bag mount with the c340, but when I rented a car I took it along and it suctioned to their super-clean window with no problem and didn't leave any marks.
On another forum people were talking about a brand of mount for the Garmin called RAM; you might look into one of these.
anyone tried both of these?
i have the 660, wondering about the 680 features.
also, anyone else have crappy clack suction cup that leaves marks all over the windshield?

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic