MMS is a no go... Tethering needs to be unlocked by VZW before it will work... VVM is a go...
Now even though it works, there are a few important things to note...
The LTE band 13 that the radio has is not fully compatible with T-Mobile - or T-Mobile is not completely compatible with it. They only have the top 10MHz of frequency in the band, so range is very limited with the M8.
The HSPA+ radio only pulls 14.4 speeds, not the 42 speeds that are near LTE speeds for T-Mobile. You will be much slower than you are used to if you had a T-Mo device.
It does not have an AT&T LTE band and same story on the HSPA+ speeds - much slower.
That said, the device has full functionality on both networks - sans picture messaging - due to the APN editor not allowing the change.
All things considered, the M8 is awesome and well worth the coin. If you were buying one outright, you can get VZW to get aggressive with their plans and save some money. I'm on a unlimited talk, text and 1GB data plan for $45 a month after taxes. Additional GB are $10 each. I'm around WiFi and usually use about 250-300MB a month, so I didn't need to get the unlimited plan, but I bet they would go up to 2GB for that price if I tried to leave.