they were the only ones who offered true nationwide service (ie, you could use your phone anywhere in the US as if you were in your home town). Everybody else (none of which exist anymore) offered regional plans. It was great and we loved it. Prices were competitive, too. Became an even better deal when we got 20% off through work. For the longest time there was zero reason to even think about switching.
But they didn't innovate and they fell behind both in coverage and in product. IMO, they are a dying company desperately flailing for customers. We finally left them 6 months ago.