I became an AT&T customer about a year ago, after making the switch from T-Mobile.
I have found AT&T to be less reliable overseas, especially when it comes to SMS. I've spent time with technical support to solve the issue, but my results are still unreliable. For instance, SMS messages either do not reach their designated party, or they don't even get sent.
I did not have those problems with T-Mobile, and I was wondering if other members have seen this.
Granted, I could use an unlocked phone and get a SIM for Europe, but it's sometimes easier to use my regular phone.
I've contemplated switching providers, but AT&T has the GSM advantage (for traveling overseas).