My guess (sorry I don't know for certain) is that one of two things will be true. Either (1) it won't work at all, or (2) you'll pay international roaming rates that will be really expensive. There's no plan you can get where you'll pay your regular monthly fee and get coverage in Mexico as well as the US.
If you're going to be in Mexico for a short time, get a pay-as-you-go USB stick from a local mobile merchant. If you're going to be there a long time, consider signing up for some kind of monthly plan, as long as you're not making an annual commitment. You most likely will not be able to use your US-purchased USB device, though. It's probably locked to T-Mobile and won't take a Mexican provider's SIM.
Buying locally, though, will always be cheaper than taking a US-serviced device into a foreign country--unless it's like a really short trip (e.g., a day or two).
