I typically use a little more than 1 GB of data each month. However, this month I am bordering on 2 GB due to testing and trying out the Personal Hotspot feature on iOS 4.3. I am going back to normal usage and the normal data plan next month though.
Since I switched from the original iPhone to the 4 last June, i've gone from using about 250 mbs to just about a gig each month. My highest ever is 1.3 gigs. Debating switching to the 2 gig plan but I'm not ready to give up my unlimited without knowing what's coming in iPhone 6 and 7...
Now that my school is finally letting iOS devices on wifi I might go down a bit.