Hmmm.... I'm on the AT&T 250MB plan, and I mostly stay under the 250MB limit unless I'm travelling. I sync calendars, contacts and outlook email between my home and corporate Outlook accounts onto my phone, check the web a bit and that's about it... I tend to use around 200MB a month, and have an auto-SMS set up to tell me if I get within 80% of the limit. I've only gone over twice in the last 15 months since I switched - both times I was travelling and using google maps a lot - and then I just changed the plan to the 2GB plan (on my phone's app) for $10 more for just that month and then changed it back afterwards.
To me, I'm happy for the choice to pay less to use less... It's saved me $205 (over 15 months... 13mX$15+2mX$5) vs. the unlimited plan that I was on.
This would work for me if they have SMS or email alerts when you get close to the limit.