Again, the phone is not totally "yours" as you are purchasing a subsidized device, which does not cost as much as an actual "unlocked" device. You can find anything you wish on the internet that will support what you want to believe.
Bottom line, in order to use the T-Mobile device, it has to be Activated by T-Mobile. As I pasted in my post above, this is clearly and unambiguously stated by Apple on the iPhone 6 information page. T-Mobile is not going to activate it just to be nice. If you would take the time to actually go to the T-Mobile site, and go through the Pre-order process, you will see that to get to the point where the device is actually activated, you have to select a Service Plan. This means that you will have to pay for at least one month of service from T-Mobile. It doesn't mean you can't change the SIM the very next day to another compatible carrier. What it does mean is that you will have paid for one month of service to T-Mobile, whether you use it or not.
When the actual, real, unlocked devices are released by Apple, they will be more expensive than the T-Mobile contract-free devices. And, again, as you will see from my post above, those devices will actually be ready to have any compatible SIM of your choice inserted into them and used to Activate them.
In the end, it doesn't matter what reddit says, or Gizmodo says, or CNET says, it is what Apple says that counts. And what they say on their iPhone information page, in writing, is what I pasted in my post above.
Cheers,
GB