When I got my iPad WiFi+3G back on April 30th, I resisted signing up for the Unlimited 3G Data Plan ($29.99/month). Since there is no contract and you can upgrade or downgrade at any time I wanted to see if I could get by on the 250MB/month Plan ($14.99/month). My thought was "why pay more up front when you're not sure how much you're really going to use?". I was right! In the first month I used my iPad as much as I needed to/wanted to on 3G. The rest of the time I was on WiFi or traveling abroad and using it on WiFi. I only used 171MB in the first month. So technically I saved $14.99 by NOT doing the impulse thing and signing up for the Unlimited Plan right off the bat. Now that's not to say that I wan't have some months where I need more 3G data and if I do, I'll purchase more at that time. However, today I feel like I just saved $14.99 by not giving it to AT&T up front.
If you have an iPad WiFi+3G and you're not going to be doing a lot of streaming video over 3G, I would advise trying out the 250MB/month plan first. Why not? You can always upgrade on the spot the day you need more. By the way, here's the renewal email I got from AT&T: