Messages has been in beta for a bit, but today Apple showed off what's coming in Mountain Lion, and I think it finally fixes a huge problem with our devices: conversation fragmentation. You may start a conversation on your iPhone, but then continue on your Mac later. In applications like Skype, or web services like Campfire, these persist across devices.
With Mountain Lion, your conversations in Messages will transfer across Macs and iOS devices. It's about time! I hope Apple provides some mechanism for allowing/disallowing conversations, however. As a parent who often allows his kids to play with my first generation iPad, it would be a bit silly to have them join in my work conversations.