No actual insight,but,just wanted to add my pre-order experience,also somewhat new to T-Mobile & 1st time pre-order from T-Mobile (pre-ordered the HTC ONE from BEST BUY on T-Mobile,but,a somewhat more definitive ETA):
I pre-ordered on 9/19,given a 9/20 delivery date (still says that,due to selecting overnight shipping),so we all know that's not happening.
When pre-ordering though,I saw something mentioning a 5-day backorder,or,something to that effect.
That is probably what others have also seen & are estimating a 9/26-ish delivery date.
I hope that's the case,but,I'm not expecting anything better than a day or so before general release.
If mine hasn't shipped by 9/30-10/1,I'll just cancel my order & pick it up locally & save the $25 overnight shipping,being T-Mobile didn't throw in any freebies for on-line pre-orders.