Pros
T-Mobile has a long history of being a values oriented company with a record of truly following through on it's "Pay for Performance" promise. While leadership changes and some mis-direction ("Stick Together" was probably a miss...) have hurt the vision and direction of the company, the majority of the leaders genuinely care about the employees and the "way" the company gets results as well as the results that are achieved. Additionally, the leadership has the capability to take a good look at it's own behavior; at the end of 2008, the senior leadership team announced that it was pulling back from it's model of large "all hands" meetings and extensive travel in favor of a "scrappier" approach to doing business.
Cons
In a word, STRESS. There is a unbelievable amount of on going stress at all levels of the company and this isn't driven simply by the current economy. Every project and effort is a fire drill and planning in many cases is non-existent. Yet when delivery isn't at 100% or quality suffers due to this behavior, there are severe penalties for failure. Simply put, if you want to succeed here, have an iron constitution and a risk takers attitude. You should also be comfortable with a lot of change, as things rarely stay the same for more than a quarter at a time.