I'm living Paris right now, AT&T service doesn't really exist here. You will jump around to different carriers based on signal strength, but it's really pretty seamless. I would strongly caution against using your AT&T sim for 3G data. You will have a pretty interesting surprise when your bill comes next month. You can go to one of three companies (Buoygues, SFR, Orange) and get a pay as you go type Sim that should be a better deal. (probably around 30 for a month with a limit around 1Gb)
On the other hand, wifi is pretty widespread in Paris, many places with free access.