I've done this with the 5s last year with T-Mobile and it was a nightmare. They tell you one thing and when that day comes, it never happens. I am not sure if they are doing what they did last year, but they didn't follow the queue of those who ordered first. People who have ordered in Sept still haven't received their orders while those who have ordered in October were already having theirs shipped. Hopefully this won't be the case for this year.
I would try to avoid ordering it through them and if you ever need to cancel, good luck. They will still ship it out regardless.