I found CamTwist to be the only reliable and CPU non-intensive solution to my needs.
I needed a chroma key feature to replace my dull office background with colourful or (sometimes) dynamic backgrounds.
I also wanted a Lower thirds added to the video.
And finally I wanted to use it live with Skype@Mac.
To solve the chroma key I am using OBS. So in it I define the camera source, background and the chroma key feature.
Then the rest is up to CamTwist. Here I capture my OBS 'output' via Desktop+.
The lower thirds are added in either OBS or CamTwist, depending on my needs, the only thing CamTwist's Scrolling text lacks is the boundaries.
The rest is already done thanks to CamTwist: a CamTwist virtual camera chosen in Skype makes the magic possible.
The only problem in the whole setup is the OBS -> CamTwist capturing done via Desktop+. Sometimes I pull other windows across OBS and it doesn't look professional. And it would be hard to accomplish without at least one extra monitor (as I have to type and do other stuff while using Skype video call).
The question... Is there any other way someone might suggest to bind OBS 'output' with CamTwist?