I still have t-mobile but I'm not ready to pay 200 dollars for cancellation yet so I'm on a 30 dollar a month plan. Edge worked great on the blackberry and I heard t-mobile has a more reliable edge network than ATT. But I plan on sticking with ATT though (and t-mobile reception worked great) because that's how the iPhone is intended to work with and I'm happy even if the reception isn't always great.
Edge on the iPhone is highly inconsistent but works suprisingly fast when it wants to. But for me, the iPhone is the first phone that feels like a portable media player first and phone second. And so far, it's worth every penny!