I have had the 60csx since it came out, so maybe 5-6 years I guess, and have loved it. As a techy person I could not find a reason to upgrade to a new unit. Even the 62 and its varients to me was not worth the money. One of the things I liked the most about it was its ability to lock on to Sat signals pretty much anywhere....
I started to look at the Montana 650 out of luck shall we say after a conversation with a friend, and it took me about two days to go out and buy one. I have not extensively field tested it yet but... It holds its position very well and does not wander as much as my 60 used to. And it still has enough sensitivity in the antenae that it locks on well. I mean well. I have been playing with it in my office sitting about 15ft from the walls and 10 or so from the ceiling.... and it locked on great, from my desk without a clear view of the sky initially. Awesome. Love the ability to use custom maps.
Now as far as the other posters, yes it comes down to your preference. Buttons or screen for the UI, and then for me as well the additional realestate that you get with the larger screen.