iMessage was a cool idea, and it helped to kill SMS. But the realization and implementation of the new Messages app in Mountain Lion is incredibly bad.
Let me give you an example: you can’t use your phone number on any device except your iPhone. This is bad. Much worse though, if you use one email address on all your iDevices, it syncs embarrassingly awkward. You receive a message on your iPhone, because you’re chatting with someone on your iPhone, and 10 minutes later, your iPad begins to receive those old messages and it does this with notifications and sounds. What the hell? Oh and if you now use Messages in Mountain Lion, it syncs minutes to catch up those old messages and it only does it when it’s fired up.
This is tragic. Not only because Apple wasn’t able to fix this since releasing Messages as a beta when they announced Mountain Lion, but because shipping this piece of really bad software in a major OS X release.