Strange you have not mentioned the fact that you cannot be sure whether the message arrived or not (well I know how to tell but none of my friends know how).
Skype is not a IM as all the others because only online users can communicate if you send message to somebody offline and you go offline they will not get it. It caused problems from my friends.
One thing I do not understand I pointed it out to them and yet most of them still use it this way.