I have been a customer for 15 years and was happy in general, but recent (Over a year ago) transaction changed my perception and I personally will be very cautious about future transactions with T-Mobile if I cannot avoid transacting.
It was unprofessional at every stage:
The order was not recorded properly and had call back after a week to reorder. Shipping label was not sent had to call to get it. TMO GRC sounds like a black hole as it could not be traced once there. Multiple calls and promises to get back with information results nothing and no call backs.
Time goes by and at the end the associate enlightened me that it is my fault to not follow up frequently. One questioned me if I am making up as they do not have a record of sending a shipping label.
If you had to transact: Keep copy of the shipping label, take screen shots of tracking, and create a folder with all the information like IMEI number, date sent, date received, etc. as you do not know what you will asked for. If you are lazy like me to keep all that information organized, run away from t-mobile :)