I think text-based synchronous CMC has the potential to be a nice medium between the "on-the-spot" pressure of FTF communication and the drawn-out discourse that occurs through drafting and revising. Video and voice CMC offer the convenience of communicating with people from around the world, but the communication itself is a little like FTF communication-- just with less clarity and often a bit of a time delay (in my experience). Therefore, I see synchronous video and voice CMC as having only two advantages over FTF communication. One is convenience and the other is to practice tasks that normally occur via voice and video chat (e.g., phone/skype interviews). It's great that we have so many synchronous CMC tools, but they should not replace efforts to facilitate FTF communication with speakers of the target language.
As someone who tends to have a high-consideration rather than a high-involvement style of conversation, I'm a big fan of asynchronous CMC. For me, asynchronous CMC allows for deeper reflection on content. I took a seminar course in my undergrad where grades were entirely based on participation in discussion board forums. Aside from the fact that I did not have internet access at my apartment, I really enjoyed learning through reading others' posts and taking the time to reflect and comment on them. I also enjoyed the discussion forums in our SLA course last semester. We never had enough time to finish our discussions in class, and the discussion board allowed me to think more about some of the questions that had arisen. I could also articulate my thoughts in a way I couldn't when I was in class. However, many of my classmates were not fans of the discussion board. Some of the posts were lengthy, and it could be too time-consuming to read. They thought it was much easier to just hash things out in class.
A few days ago I commented on our CALL class discussion forum that the decision to incorporate synchronous vs. asynchronous CMC depends largely the feasibility and the particular language learning goal, and after reflecting, I think there are more considerations to add to this list, including degree of convenience, degree of authentic language use, and individual conversational and learning styles and preferences.