I'm not a techie, but this is a cool app. Here's what I like.
1. You can create a group chat, where everyone is chatting in the same window and can see everyone's comments. I use this with the pastoral staff to set meetings and to solve issues that involve multiple people.
2. It recommends friends to you. You don't have to go hunting and requesting friends. As soon as someone in your phone contact list becomes a Kakao user, the system invites you to add them as your Kakao friend. This is hugely convenient.
3. It's free - almost. You have to pay for data transmission if you're not in a wifi zone.
4. There's no character limit. Your messages can be as long as you want.
5. Virtually every smart phone user in Korea is on it. That greatly reduces texting fees.
You can also share photos, videos, files, and links, but I don't really do all that stuff. Like I said, I'm not a techie. I use it for simple communication - kind of a hand-held modification of email, texting, and chatrooms. You can download it for free at the istore or android market.