Looks like Yahoo! released the latest version of Messenger. I've been using the beta version for awhile and found it to be pretty reliable. The biggest feature of this new release is what they call PC-to-PC calling. Essentially, this is like a voice chat but if the person on the other end is not around, you can leave a voicemail. While this feature itself doesn't really seem all that revolutionary, the voice quality in the beta version was excellent.
Some other nice features is the file sharing where it now supports drag-and-drop. Of course, there is the many integration to Yahoo itself and existing features like photo-sharing, etc.