I have absolute sympahty with the OP. iMessage is completely unreliable. Only today I sent an iMessage to a friend, it didn't zip through and the text message then failed to even show up in the list of SMS, gone forever, vanished without a trace!
MMS will not send outside of a solid Wifi or 3G signal. If I send a picture MMS on a non-3G or WiFi signal or I drop out of signal before it finishes it fails to send and will often completely stuff that message on the phone up until I do a hard reboot of the device.
Often messages show up as not delivered when they have been, and I've known of times where a message reported as delivered from a friend and I never got it (although I only know of that happening once, once was enough to not trust it though)
I've turned it off and won't be going back to it for a time, to some degree, with the amount of free SMS you get iMessage is a little pointless without mountain lion anyway ...
Luke