... All the development will be focused on the 550 as it's bugs.
I'd give tomtom another chance. I've heard that now you need a tomtom app (mydrive?) on you phone instead of the default bluetooth tethering since this has been problematic on recent phones that seem to disable the tethering option at will. It might be so that you can chose to tether without this app as well.
No built in SIM, There having a laugh, its one of the reasons I didn't buy a 450. Missed opportunity again !!!!!
Is adding SMS capability and more connectivity not just adding to the elephant in the room that is 'distracted driving?'