The great thing about the new iMessages app on these iDevices is that you do not need cell service to communicate with another iDevice. iMessages can communicate simply over a Wi-Fi connection. This is what allows the iPad to talk to other devices via a "text message" since the iPad cannot technically send a text message like a normal phone, even if it uses 4G LTE. This is also applicable to the iPod Touch as well. Now, with the addition of Messages to OS X, the MacBook is now included in the family of iMessage-capable device. The interface of Messages looks similar to iChat (it still allows you to include accounts from AIM, Yahoo, Google Chat, etc.), but it also allows iMessages to be sent from the client. Messages is much more reliable and useful than the old iChat.
Keeping Things Organized
When I was using the Messages Beta before Mountain Lion was released, I had to deal with a lot of bugs. I had included my Facebook account within the application too, so I could send and receive Facebook Chat messages through the application as well (which is a lot less distracting than having to log on to Facebook in order to send a chat). The most irritating thing about the Beta was the lack of communication between devices. When a message was read on my phone, it was not marked as read on my computer. This drives people like myself, who are particularly anal about having notification icons sitting around on my screen, crazy enough to make sure all of my messages were marked as read all the time. The sad thing was that I continued using it just because of its convenience. Now with the full release, all devices using Messages now speak to one another and make sure that all of the new messages have been marked as read. This even clears any notifications on the lock screen on my phone (which I was pleasantly surprised by). If I receive an iMessage from another iDevice and I read it on my laptop first, then the notification on my phone and iPad are marked as 'read' and do not appear on the lock screen as another annoying notification. Each of the individual applications also update upon being opened so that they contain all of the recent conversation that may have been sent from a different device. This was you can easily continue conversation from one device to another.
"Enjoy the Little Things"
Messages has a few additional Easter Eggs that make it even more pleasant to use. Read Receipts are similar to something that chat clients used to use back when AIM and Yahoo Chat were popular. iMessages allows the user to see when the recipient has Read the message and then shows an animation that lets you know when they are writing back. This can be useful for something like a text message, so that you know whatever you were trying to tell the person on the other end, has reached them. Read Receipts are something that can be somewhat bitter-sweet (i.e.your naggy girlfriend knows when you're not replying to her text messages), but despite the lack of privacy, it makes life in the world of electronic messaging much easier and much more usable.