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I picked the 550 up on eBay for 615 a few weeks ago. I love the way mine works and performs. There are some features that take a little getting used to but over all I love it.
I have a 450 and it works very good. The guy thats riding with me has a Nuvi 760. So far it has proven to be a pita, can't see the screen and it actually routes a little different than mine. I could have got the 550 when I bought mine and now wish I had. If I were you I would get the 660 from Amazon.com and not worry about weather you got the right one or not.
I am not sure whether the difference is in the actual unit or the different mapping versions but I noticed the exact same thing. I have a 450 on the bike and a 765 that I bought for the truck and to take with me on the road. The 765 route calculating is odd at best. The Zumo is painless and I am not sure why.
