I like the simplicity of the Cel.ly interface, and it seems to work pretty well across both web and mobile clients. It's interesting that you can have point-to-point conversations where you are just talking with one person via direct message (and have push and email notifications apply the same).

Unfortunately, I don't see where one has the ability to start a temporary group conversation. This would be something I'd want if, say, I wanted a message to go to a subset of people, and have the potential for that conversation to remain among that subset (say, GTA5 bat signals, or whatever).