DEPENDS ON AREA >>>>
The Verizon M8 has HSPA+ 21 radios in it that work on the 850 and 900 bands for voice and data. This means in MOST areas you will get 3G and H+ data using T-Mobile. If you live in an area that T-Mobile has run out the Band 13 LTE connections, you have about a 70-30 chance of keeping that LTE lit up. Major cities should all have the Band 13 lit by now, but if not the HSPA+ speeds should be good. Just watch for low signals indoors that will crush your battery life.
Other than that, it seems to work very well - I actually turn mine to 2G when I am indoors or on Wi-Fi as it saves a ton of battery life.