I love the flexibility that CamTwist gives users who might not have the largest video production budgets, but still care highly about production quality. I use CamTwist with LiveStream and BoinxTV and it fills roles for these quite well. I used to use it with Skype, but I am sad to say that- with the newest version of Skype- it does not work anymore.
Overall, CamTwist is a neat program, though a bit rough around the edges- the "Studio" interface could be improved- the buttons can be difficult to move around; and the video mixing environment could be better (but hey, that's what I have BoinxTV for). I would be willing to pay for a more polished version, but for a free version, it does rather well (I just wish it still worked with Skype).