Pros
When I was there, the pay was good (perhaps slightly lower than at competing companies like AT&T or Verizon), the atmosphere was relatively relaxed (I hate high-pressure/high-stress sales), and the company was good to the customers for the most part. I was very happy with the benefits.
Cons
My immediate management (store and regional managers) were frustratingly unhelpful. I wanted to move up in the company, and was met with resistance and negativity. This strikes me as mainly a local problem, though, not company-wide. I was starting to get very frustrated by the frequent changes in the benchmarks they wanted us to hit. Our goals kept shifting and morphing, making it difficult to keep up with what they wanted of us. When I started with T-Mobile, they just wanted us to sell phones. Then they started watching metrics. Then they couldn't decide which metrics were important to them. It was a pain.