This summer I dropped Verizon to try T-Mobile's North America phone plan. (Canada, US and Mexico)
When I signed up for the service the T-Mobile guys didn't have all the details on how well it would work in Mexico or what partner cellular network was used.
Now that we have crossed the border here are my findings.
I'm using an HTC android phone and after crossing the border at Tecate the phone went into 3G mode and no data functions worked. I was able to make outgoing calls to the US though.
After playing around with different settings I discovered that roaming was turned off. Once I turned it on..... everything worked fine. 4G network from Telcel was discovered and data functions worked just fine.
I have a Google Voice number and call forwarding is working too.
I cannot call out using Google Voice. (didn't expect this would work)
Turning the phone into a hotspot and tethering a computer works too. The plan I have limits the tethering data to 7GB in the US. I suspect the same is true here but I haven't tried to hit the limit.
Outgoing calls to the US require you to prepend a "+1" to the number.
So far so good.