I have found myself recently trying to get people to use Skype for skype-2-skype conversations. Now its not as easily accessible for those with limited bandwidth, firewall issues, or limited hardware (like no mic) but those issues are becoming more and more non-issues.
The time I can take to say something and have a truly dynamic conversation vs. typing is rather small. And unlike a phone, I don't have to cramp my neck into weird positions and use only one hand. Also the screen collaboration plugin (YugmaSE) is awesome. IM just seems to be more of a pesky distraction more than a means of communication now.