I've had a 550 for 7 months and yesterday I returned it for the 670 and glad I did.
I would describe the 550 as the "Paris Hilton of Remotes". Comes from a good family, looks very good, but once I got to know it better it really started to drive me crazy. The button size and layout was such that I never did get comfortable with making selections without first looking to make sure I was on the right button. In the end, the volume down began to stick on me and Circuit C gave me a store credit which I used toward a 670 (rebate this week on the 670 is $60 so it will end up costing me less. Also, kudos to CC for this no questions asked transaction).
My initial impression of the 670 is great. It feels good to hold, for the most part the buttons are well placed and have a solid feel. And of course it controls everything in my system perfectly, just like the 550 did.
My biggest concern in making the switch turned out to be a very pleasant surprise. I just knew I would have to go through an entire new setup with the 670, but the Logitech software allowed me to transfer all the settings I had for the 550 to the 670 with only one button needing to be re-mapped. The "Prev" button didn't do anything so I just went back into the software and mapped it correctly. The change over took about 20 minutes including this fix.
I can find only two things I need to tweak. First, since the 670 doesn't have a DVR button (still don't know why Logitech left this one out) I have a soft button mapped to "PVR" but will be looking to see if there is a better hard button to use. Second, the "Sound" and "Pic" buttons currently serve no useful purpose for me. Unless someone can tell me their real purpose I'm likely to use one as the "DVR" button. Suggestions?
Setup: Panasonic 42" plasma, Dish VIP622, Yamaha Receiver, Samsung DVD