When iOS 5 came out, I logically turned on iMessage on both my iPhone and iPad. What I liked with iMessage was not being able to send free texts (I have 500 texts included in my plan for free and rarely use more than 50-100 a month so cost is definitely not in the equation here), but rather the idea of having my messages delivered (and stored) to all my devices seamlessly.
Well, at first, my iMessages were not delivered to both devices but only on the iPhone. We later discovered we had to set the callerID to the email rather than the phone number, that should work. Well, that solves the problem is someone iMessages me to my email address but if an iMessage is sent as a regular text to my phone number, I still only receive it on my iPhone.
The major problem so far, however, has been reliability. I don't seem to receive every message sent to me. Also, I often sent a message to a friend that seems to be routed via iMessage. The message seems to be sent, nothing comes back (the user might not have turned on the "Send Read Receipt" setting). However, 30 minutes later I receive an alert from my carrier (I am on pay-as-you-go hence the alerts) that a text has been sent. I go to my messages and discover that the message has been sent as Text Message. I'm ok with that, the only problem is that it took 30 minutes for my text to reach its destination without myself knowing about it. This is definitely a problem for me. I use text messages for fast communication with friends and I really expect any text I send to be received the second it leaves my device.
I have disabled iMessage for now. I will keep looking at it and I can wait to enable it back, but it seems not reliable enough at the moment.