If your browsing the web everyday, even heavily then 1gb of data is perfectly fine
I disagree.
Just activated my "unlimited" plan, and based on some preliminary testing, I'd say 100MB/day for LIGHT browsing!
I'm going to leave my WiFi off for at least a few days and see how it goes.
My usage is pretty light. I scan the news in the morning - USA Today, Bloomberg, then Cnet, SlashDot, and MacRumors. Maybe listen to the latest NPR newscast. Perhaps follow some links from the above.
During the day, I might use it to read books (but no data usage there), maybe to look up a few things on Wikipedia.
I typically do NO streaming of any kind. Maybe a particularly interesting news story.
I think my usage, if completely "off WiFi" would be 3GB/month.
Now, this is unrealistic for me, right now. I rarely take the iPad out of the house. (Though I intend to use it in the car now.) I'd be on WiFi except for this test.
But, for example, I used to work in an office where connecting personal equipment to the network (Eithernet or WiFi) was strictly prohibited. And I prefer to keep personal stuff personal. So, I'd stream music on my iPhone to Bluetooth headsets, and use the iPhone for any personal business during the day.
I also occasionally brought in my MacBook as an extra screen to read technical literature, but without being able to connect it to their network, was not useful as an extra web-browser screen.
If I had that job now, I'd be using the iPad from work every day, and probably rack-up 4GB/month or more.
I can't imagine what a family with a pack of kids in an SUV is going to use!
Oh, you need to add the download and upload stats. It's not just "download" that you are paying for.
iPad usage will be much different from iPhone. You are going to be using the browser more, and most sites are going to be serving-up the full kaboodle, rather than some scaled-down iPhone-specific content.