Well. I'd recommend it. Especially if you are sick of paying to much. You have (unless in CA) a 14 day buyer's remorse period. I'd test the phone in all the most frequent places you go to and make sure you have the service you want there before you end the 14 days. This is important with all carriers. If you have 3 bars or more i'd say you're solid. Be forwarned though most all of upgrade policies work close to the same as other companies. T-mobile's is every 22 months you get a "Full price upgrade" or "best priced" During that 22 months wait if you need to get a unpgrade you can get partcial one. Its usally a small discount but better than paying full price which you can do at anytime after you do a full price upgrade. You can port your num...
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They use a higher frequency than AT&T and therefore don't have as much penetration in buildings and such. I would be sure that their service works everywhere that you need it in your 14 day trial and before you port your phone number over. If they work where you need them, T-Mobile is a great company.

Hey, I had TMO and they do have great customer service and great pricing and their coverage is pretty good for my use. But I just got sick and tired of not having the option for 3G. And even as TMO has recently rolled out their 3G and say they are going to continue so, it still only extends just to the South of me...and I live in the 3rd largest city in the NW.
Plus TMO's freakn' 3G band is off the wall (sloppy seconds) therefore your phone selection is limited if you want a TMO 3G phone.
Check E-bay for the ATT phone you want and see if it's there cheaper.