If the acquisition moves forward, it definitely has a significant impact on the level of competition in the mobile market.
T-Mobile also has great customer support and serves my unlocked Nexus S without a contract and with free tethering, something I'm sure AT&T will refuse to do.
PS: I used to have AT&T (actually cingular back then). Service in Chicago was atrocious. Any phone call over a few minutes would drop at least once, I'd have to stand in the window of my apartment to make phone calls, etc. Once again, YMMV.
In DC, all the services were down for about that long.
T-Mobile's coverage in DC is indeed quite spotty: at GMU where I work it's quite good. But at my home in Alexandria, it's far from hot.
But for me, there is one reason I use T-Mobile. The Nexus S. An unlocked, no-provider-junk, Google-updated phone which is updated 6 months before other phones and which has 3G support in Europe. [The last bit knocks out AT&T and Sprint's version of the phone, and it's a big deal for me]. Just wish T-Mobile would permit UMA on the phone.
Carrier diversity does have its advantages in other ways. I was in NYC during the earthquake and had no problems with text or data immediately afterwards, even took an incoming voice call. Everyone around me with iPhones on AT&T or Verizon couldn't do a thing.
Not to mention, having only one GSM carrier in the US is stupid.
I had a great deal while on T-Mobile and it was very hard to quit them. Personally, I believe that competition equals innovation which is better for consumers, and we need more of it. The situation for mobile service is quite sad here in the States with the collusion of the big two(not counting T-Mobile and Sprint, I suppose) on pricing and such, and having AT&T get even bigger wouldn't help the situation.
My company offers iPhones, so I really only keep a non-business cell for sms and an occasional call. So far, T-mobile has been great for that (in Houston), and I am confident my monthly price would increase - perhaps even double - under Att.
Reception has been somewhat poorer since I switched from AT&T, but price and customer service and much, much better. In particular I appreciate being off contract.