Messenger is a worthy challenger.
We found Messenger's voice chat relatively simple to use. We just right-clicked on the name of one of the people on our list of friends and selected the voice chat option. A window popped up and connected us to a voice chat server, letting us begin our conversation. The chat feature is full-duplex, allowing you to talk and listen at the same time. The sound quality was fairly good over our DSL connection, although understandably it was not quite as good as with a regular telephone connection.