So I did it--just pulled the trigger on the 630-->670G deal this afternoon. One contributing and not altogether rational factor was the convenient/inconvenient fact, depending on how you look at it, that I did a sensor replacement yesterday and got the Dexcom low-battery warning saying it won't let me start a new session when my current sensor runs out, now in less than a week. Since I hadn't yet spoken to M-t I went ahead and had my G5 scrip refilled, figuring that however I decided I was definitely going to need Dexcom supplies because it will be at least a couple of weeks to get all the bureaucratic/insurance stuff cleared before a new pump can arrive.
Which means I'll wind up with at least one unused G5 transmitter still in the box plus enough sensors for several months (more stuff for my Zombie Apocalypse Drawer!) As it happens my G5 receiver cratered yesterday as well, so I've got one of those coming too. Slightly irrational but it kind of eased my mind that if the new system drives me nuts I'm well equipped to revert to status quo ante, at least for a while, so when the M-t rep called I said screw it, let's make the leap.