I wouldn't say T-mobile is slowing down. Their data speeds are very fast (30-40) in good coverage areas, and thats the problem the coverage areas. My friend has Sprint and there are plenty of places he gets service and I dont, especially in certain train stations he's able to get calls and text and my phone has no service or maybe half a bar.
I live in Brooklyn so that's where I'll use it the most I guess, but I work in Hoboken NJ and hang out alot in Manhattan, (SOHO, Midtown) on the weekends.