Each data purchase gives you a new 30 day period in which to consume the data.
No additional fees are tacked onto the plan cost.
You are billed every month. If you go over in the month and elect additional data you have only until the end of the current month to use it.
Additional fees are tacked onto the cost similar to those extra fees on your wireless phone plan.
For the 250MB plan the fees in my area are $1.68 on top of the $14.99.
I don't have personal experience with this, but I understand you can start/stop/change your postpaid plan anytime. Example: if you have the 200 mb plan and see that you're going over, you can change to the 2gb plan and get another 1.8 gb for $10 instead of another 200 mb for $15. Then, when your next billing cycle starts you can go back to the 200 mb plan.
Something that I've noticed is that my iPad transmits large amounts of data (~50 mb) in the early morning hours when 1) it - and myself! - are in the sleep mode, and 2) I'm at home on wifi. S-o-o-o... my new policy is to keep cellular data turned off except when I'm using it.
I got my iPad from an Apple dealer and took it to an AT&T store where they added it to my AT&T account - no muss, no fuss...
Convenience, maybe. You have to call AT&T to set up the postpaid plan - they run an instant credit check. Then there's the odd little issue of the password if you want to manage your account online. A cell number is assigned to your iPad and they want to text the password to that number. Problem is the iPad's not a phone and doesn't receive text messages. So it's snail mailed (takes up to ten days), which I still find hilarious in light of the technology being enabled. Ordering the service directly through your iPad is quicker and if you stay within the limits of the plan selected, going that route makes sense.