Been reading this thread since the beginning, decided to post my experience for what it is worth.
Went from an iPhone 5 on Verizon to an HTC One M8 on T-Mobile last May (they paid my verizon early termination fee)
I've had iPhones since the 3gs, and I work on a Mac computer, but I thought I'd give Android a try, cause the M8 is really a sweet phone...
What a mistake! I was assured that, with the Jump service, I could upgrade when the 6 was released. Come to find out, someone says I declined the Jump service, it was never activated, and now it is too late.
I was, rightfully, outraged.
Anyway, it doesn't matter, I am just going to sell the pristine HTC on the auction site and be done with it, but this was the first time T-Mobile screwed me.
I preordered on 9/14 (NON JUMP, obviously

)
Backordered on 9/19, had a pending charge for $1, got all sorts of texts, the last of which was "important and prioritized"
Friday 9/26 my AMEX was charged for ~$186, that charge is still pending as of this morning.
New backorder date of 9/28/14 @04:11:41am
no shipping label, no nothing. just says UPS and backordered.
6+ 64g Gold