What would happen if I got this?

I'm trying to get an iPhone without the phone company putting me under a costly data plan. If an iPhone is factory unlocked does that mean anyone can use it as long as it's the same service provider (i.e an AT&T iPhone--factory unlocked--when I have AT&T)? Also will it just allow me to use whatever I already pay for as far as capabilities? For example, I have the 3G Service, and then the general texting and calling functions. Will that stay?

Most if not all carriers require a data plan with smartphones regardless of if it's unlocked or not. The costly data plan is nothing compared to the bill you would get if you didn't have the plan.