I use that in my iPad. Works like a charm and automatically steps you up to a higher plan depending on how much you go over. Not bad at all.
Example: I go over my 1GB plan. Overage is then 1 jiao per MB. I get charged at that rate until I hit 1.7GB (80 for 1GB + 70 kuai worth of overage) whereupon it automatically steps me up to the 150 for 3GB plan. Haven't gone beyond that yet.


One more thing- if you buy from Unicom you have to buy a certain amount of airtime with it. Unicom stores have a modem+card option where you still have to prepay 260RMB of airtime with a modem purchase. However, if you already have an unlocked modem and just want a card you have to go to the electronics markets to get a card only.