I certainly can't say anything that hasn't already been said about AT&T's new cellular data plans.
What I can do is show how it affects me and let you see how you can determine if it affects you.
I went back through my AT&T bills for the last year. Here's where the data usage shows up in my bill. Notice that mine is a BlackBerry feature code so yours may be different.
The simplest thing is to search for "kilobytes".
Here's my data usage month by month.
Clearly I'm way under 200MB per month. My BlackBerry data plan is $30 per month (actually $35 with 200 SMS messages) so it looks like I could change my plan to "DataPlus" for $15 and save $15 per month. Remember that you can't go back to the unlimited (actually 5GB) plan.
There are a couple of observations from looking at the data. My data usage didn't change when I went to 3G with the Bold. My conclusion from this is that the screen on the Bold is still so small that I'm not willing to surf very much on the phone. A second observation (really a question) is why are those two months (June and August) twice the rest?