A friend of mine had the 660 for a couple of years and then bought the 350. After using the 350 a few times, he only uses the 660 now. I offered to buy the 350 from him and he said he wouldn't do that to me. He said it works fine, just a lot more steps to get info in and out of it and move from screen to screen...a lot less "elegant" and somewhat difficult to use. It now sits on his desk as an expensive paperweight. I've also read that the trip planning software it uses is very unfriendly and is almost a DOS program.
I just bought the 660 and so far it's very similar to my old 450 (with more bells and whistles), and easy to use, so I'm happy I went that route.