iMessage is probably one of the most notable and widely used new features that Apple introduced with iOS 5. ‘Free’ messaging to anyone else with an iOS device without having to think. But it was fiddly; it required effort to set things up to get them as Apple described; the ability to sync messages and pick up where you left off on your other iOS device (and now, your Mac).
There was also that bug where an iPhone could continue to receive messages for a number associated with a SIM card… that wasn’t even inside the phone anymore.
iOS 6 finally attempts to fix both of these annoyances.