I know what you mean. I have an unlimited plan that I don't want to give up. I only use around 1gb/month, but I don't have a 4G phone yet. That might change things.
My two daughters (15 & 18) have been begging for smartphones, and I've been waiting to see about this data sharing since last summer. Finally decided I'd let let them have the "300mb for $20 plan" at Christmas. I basically set them up with WI-FI Sleep Policy to Never, Update over WI-FI only, installed the Data Use widget, setup Data Usage text alerts at 50% & 75%, and told them any overages come out of there allowance. One month in so far and all is well. Time will tell. I don't know how well this would work with a shared data plan though.
-Wes