I have am AnDroid phone on the AT&T network. Until I figured how to turn off a lot of the data features I had used up half of my 200 mb data plan in less than 2 weeks. By month's end I figured out what features were draining the data and battery use and tried to be sparing until the new billing cycle began. Now that I know how to turn those features on and off, and figured out how to set up the wifi at home, I use way less data.
So, if you don't have access to wifi, you will probably want an unimited plan, if you want to take advantage of all of the cool features that come with an AnDroid phone.