I have a 550, and am very happy with it. I don't remember which model I got first, but it was very uncomfortable - hard plastic buttons, no soft-touch covering on lower case. The 550 is slimmer, has soft buttons, and soft, rubberized coating on lower case.
I made my own "device" for the Apple Remote, before Logitech added it to the library - works just fine.
I guess this really didn't answer your question, except that I don't think there's any quality issues with this model.