Facetime only works if both iPhone 4's are connected to wifi. A Cydia app called 3GUnrestrictor makes the phone/program think it's on wifi, so it's data will be sent, but via the 3G network.
OP, I'm a light to medium Cydia app user, mostly interface changes and UI enhancements. I use Winterboard (themes), SBSettings (easy settings), Activator (more swipe control), OpenSSH (to do it all), Backgrounder (extra enhancements over the built in), PrivaCy (stops some of the automatic data upload that various apps may collect), Talking Alerts (changed sounds for voicemail, email, text), mCleaner (call blocker), iFile (file manipulation, etc.), WiFi Sync (sync without connecting to your computer), and Remove Recents (removes all the apps in your Apple generated taskbar).
Biggest thing to remember, is that you when choosing Cydia apps, you do search to see if others are having issues with the app BEFORE you install it. Some Cydia apps get hurried bug fix releases every now and then, and will cause you to have to restore your phone. IOW, don't be an early adopter with any Cydia app, and you most likely won't have to go through that.