Rollover is cool. But because of their expensive plans, casual users often end up with lots of unused minutes. Since they added free calling to other cingular members, our family plan has had over 10,000 rollover minutes (at one point).
It's sort of a another way Cingular eats your money. =/
That's interesting, because Cingular (at least at the time) was also the cheapest carrier for me. At least for a two-phone plan (sharing minutes).