If you get an iPad 3G and are faced with the choice of buying a 250MB or unlimited data plan, my advice is to go for the cheaper 250MB at $14.95 per month. If you find that you use up the 250MB in less than two weeks, and it seems like your usage will continue at about the same level, then when your 250MB runs out, go for the $29.95 unlimited.
But in every case if your 250MB lasts longer than two weeks, you're going to be better off buying 250MB at a time.
When your usage hits 80% of the 250MB, you get an alert from your iPad. I got such an alert last week, about 20 days into my plan. It was obvious that I should go for another 250, which is what I did as soon as I got the alert. But note that even though I've already ordered another 250MB, that order won't be executed until my current 250MB runs out.
If you go for 30 days and you still haven't used all of your 250MB, it doesn't roll over. At the end of 30 days you'll have to spring for another $14.95 or face losing service. It's set up to bill your credit card automatically unless you cancel your service.
I currently have 16MB of my original 250MB left, so it looks like I won't make it the 30 days, and my next order will execute before the end date of May 30.
Of course, there are techniques you can use to try to make sure your 250MB allotment lasts the full 30 days — and you can read about that in a helpful article on the Computerworld website.