Well, just looking at my account, I've used 140ish with three day's left in the billing cycle. Buy I have only had the Captivate for 7 days. Before that I had a Blackjack. I remember the first month I had the BJ I accrued something like $6,000 worth of charges (they put the data on the wrong line at the store) can't remember how much data that was though... My usage dropped off sharply though. It was nice having the option but it was kinda cumbersome. Last couple of months I only averaged 33mb. Before I was averaging closer to 60 with a peak of 170ish. My brother, who has an iPod, is usually between 200 and 500mb. Last month he peaked at 1.6GB which was most likely due to World Cup streaming.
A friend who has an iPhone says she rarely jumps above 200MB. The nice thing about the 200MB plan is that if you go over they just tack on an extra 200MB instead of normal overage charges. Yes, its and extra $15, but $15 once in a while isn't going to hurt anyone. But if you continually go over it, or go over $400 it could end up costing a lot. I'm grandfathered into the "unlimited" (5GB) which is by far best deal, but it would be hard to continuously go over 2GB unless you're tethering a lot and never on WiFi.
[Edit]
Ok, looking again: since I got my Captivate I have used 126MB (in 7 days). Over a course of a month that would be 543MB, but I've been putting it through the ropes. Something more telling is my brother's trend line over the last year or so.
.1 .2 .1 .1 .1 .2 .5 .4 1.6 .3GB. I will probably use more than him but I am on WiFi at work, home, and all my friends homes. I think it just shows how quickly and easily you can go over 200MB.