When I talk to someone on T-Mobile, I can't understand much of what they're saying because they have horrible voice quality. I experienced this with their predecessor VoiceStream. I don't make many voice calls but I would like to be able to understand the person on the other end, and for them to be able to understand me.
As I mentioned, T-Mobile is awful here, so I would have to be on Sprint's network to get good service. Every time I ask a local T-Mobile user about their service, they can't tell me anything good.