T-Mobile roaming active for me, started today, in Maine.
Fwiw, it wouldnt be a sprint change / enhancement if it didnt cause me some minor annoyance that I need to deal with. In this case, when connected to t-mobile lte, it seems to be running on GMT for time so the phone rolls ahead 4 hours causing issues with texting (and probably other stuff). Progress I suppose. lol
EDIT- I did see some quick connections to band 12 as well.