I'm not sure which provider you have, but if it's Verizon you need to make sure to see if you're grandfathered in with unlimited data. About a year ago Verizon did away with unlimited data, so now if you're grandfathered in with unlimited data and you want to upgrade to get the cheap price of a phone by agreeing to a 2 year contract, you have to switch your plan which means you no longer have unlimited data.

Since I learned that I made it a point to use as much data as possible, so I use on average about 15GB data monthly. I think it's crap that they force you off your plan and they try to say it's to save you money, but it's a load of crap because the plan ends up costing you more if you use 4GB or more a month.