AT&T's system for international add-ons on their site is a lot better than it used to be when you had to explicitly cancel the plan over the phone when you got back home.

I'm happy enough with their offerings. I suppose there are circumstances where I'd just buy a SIM card for my old phone when I arrived at the airport, but for most purposes I'm fine with paying $30 for 120MB or whatever it is. It's no hassle and serves my purposes for a week of travel or so.