People talk about it 3g being too much but all you have to do is be realistic about it.
Try out 250mb a month and track your usage. That's 15 dollars. That's a lunch or two. Make a sandwich, eat cereal and leave mcdonalds and ihop alone. Probably better for your health too.
And you got yourself a good 20+ bucks. Plus, there is absolutely NO contract so you can stop any day. Money is tight, stop 3g. Things are good, add it back on. Simple.
Use wifi when available and 3g when needed and maybe you'll be under 250mb.
People complain about at&t and taking money and this and that.. but think about your life and all you spend on other misc stuff. It all adds up too. I know people spending 40 bucks on a gym membership and they go like twice a month.... like that makes sense?
Ideally, with an iPhone 30 dollar unlimited plan, a 15 dollar add on should get you unlimited iPad coverage, but that isn't the case. My logic is, I'll only be using 3g on the go. That's in the car (not driving of course), waiting for appointments, etc. Those times, I'll probably be surfing the web, email, twitter, social network checks, etc. Movie downloads, video stream will probably be under wifi anyway. I don't think people I'm with will like me watching a movie while I'm around them... For my use, 250mb a month is plenty to go along with my iPhone plan.
Like other people have said, it's not like I'll be walking down the street and busting out the iPad to update my facebook account. This is more for leisure sit down times when you wanna pass the time. iPhone is quick on the go ready at any moment device. iPad is luxury. Sit, relax, and connect. Enjoy.
Since owning the iPad these are various places I wish I had been connected...
At the movies waiting for a movie
Lunch break down the street from work
Long car rides on the weekend, visiting family - not driving
Traffic - not driving
Waiting for appointments
Cabo airport, wifi was not free
During a wedding reception (hah!)
And this is like in the last 9 days...
Plus, going to the iPhone after iPad usage, your iPhone feels dinky and outdated. My once, treasured iPhone now feels dinky lol.