I found over the past two months that I was quickly reaching the 250Mb limit on my AT&T 3G iPad. I have a post-paid account so I don't get the online option of quickly upgrading to the higher limit. I use WiFi at home and when it's available, but use the device quite a bit while on the road. I've now changed over to the 2Gb plan.
I can't imagine getting by with only WiFi. My wife was doing work-related research the whole time we drove across several states last weekend.
How's everyone's usage working out?