Agreed with Deaconclgi. I was former 6years At&t user- in my location the reception was ....- I had to go outside the house to make a call. Switching to Tmobile (because of N900 of course!) does not require me to go outside, when it is raining or snowing, only when the weather is nice outside!
Have a friend who has both ATT and MetroPCS (Staten Island)- he was surprised that "crappy" MetroPCS works better in his place than ATT or my Tmobile.
I always use Skype on TM 3G when at work (New Jersey area)- works with no problems, no drops or something. Compared to some IPhone 3gs- the same pages load quiet faster on Tm N900 3g than on ATT 3g 3gs