What is the catch with the Virgin Mobile iPhone plan?

I mean, an iPhone 5 for $30 bucks a month and no contract? I don't feel like drowning in hidden fees, so can someone tell me what the catch is?

Oh, and I know I'll have to buy an iPhone 5. I'm just wondering if there are limits like "$10 for every mb over your data limit" or something. I've never owned a smartphone before, and this sounds perfect, unless there's a crippling catch.