I have a oneplus on Tmobile, and the phone is great, the service is great, the two combined has a few weirdnesses in it.
What i have found is that the oneplus has one weakness, its radio.
While plenty powerful it lacks the 700mhz band that tmobile is focusing on for its LTE expansion. Add to this that the Oneplus only shares one of the 4 total frequencies that tmobile uses. There is the same issue with AT&T.
in most settings this is not an issue with the exception of every now and then running across a weird deadzone that only seems to affect you and none of the other tmobile devices nearby. it also really showed up when i drove to snowflake (from Tempe). I only had service for about half of the total trip while my wife with her phone had service the whole time with a couple of moments interruption depending on where the mountain peak was.
Since then i have dropped CM11s for a late dec CM12 nightly that someone is compiling and sharing out, and then amusingly enough putting the CM11s (most current) radio back into place and right now this is giving me the best connectivity so far in the valley and i am not having the issues with deadspots i most normally have run into.
but the device is amazingly powerful, well built, and for me the best part yours. you get to unlock it if you want. you can rom it if you want. entirely up to you.
Also get a tempered glass screen protector at least, the way the edges are exposed, while sexy, makes them vulnerable to nicks and chips (my screen protector has eaten one of these moments already)