I have been with all of the major carriers, and even some minor ones. My last couple being Cingular and Virgin Mobile. I needed a phone and was hooked up in the store within minutes, the staff was very friendly and helped me get the phone I wanted.
I've been a customer for 1 year now, and I've had a few annoyances with them, a few times I had issues that I thought they should resolve in a certain way and they didn't, but other than that the 611 call center has always been helpful and quick in assisting me, and the stores in Panama City have been very polite and friendly, whether I'm returning an item, purchasing a new one, or complaining about my bill.
My only issues with the stores is the customer service, when not focused on you, can be lacking -- it's typical to wait 15 minutes in line with only 2 other customers in the store. The problem is that the T-mobile reps haven't been trained to multi-serve -- so they will gladly sell the current customer a plan, help them explore the store, and give them the attention while you wait 15 minutes to just be rung-up on a charger.
Unfortunately this isn't T-Mobiles exclusive fault, this occurs with alot of the other stores, regardless of which service -- they make you feel as if you are waiting for a rep on a car lot. T-Mobile fail at this worse than the other stores, but I don't think it merits losing more than a star.