It's really unreliable and half assed. The parent commenter is a absolutely right. iMessage is full of bugs they need to really fix them! They have done an amazing thing though.

Sidenote: apple has the ability to mine so many conversations for data. It's more intimate than twitter and I'd imagine more iMessages are sent each day than tweets. If something happens anywhere in the world Apple would theoretically be the first to know. Kinda nuts.