I've been using this setup for about a week now, and I have mixed feelings.
I usually get about 8 mbps to 12 mbps downloading which is fine. However, T-Mobile's upload speeds are just awful. I am only getting at max 1 mbps. So if you upload a lot of pictures to instagram/facebook or videos to youtube then I would not recommend T-Mobile. Verizon has really good upload speeds, from my tests they're about 12 mbps to 14 mbps. Which is perfect for uploading media to social networking sites.
Using my iPhone with Google Maps while drive isn't a problem. Now using Google Voice to make VoIP calls while driving that is a different story. I can hear the other person just fine, they just can't hear me all that well. Now if I am in a store and I need to make a call via Google Voice then it's not a problem. Using Google Voice to makes calls at home via my home wifi connection isn't a problem.
What phone r u using nd where r u located?