I am using this on a Mac Pro running Leopard 10.5.6 and using a Logitech QuickCam Vision Pro.
Works well with Skype (beta version 188.8.131.524) so long as I configure everything such as the effects I want to use, before launching Skype. It's a shame you can't change video effects during a call, for some reason it doesn't work - I have to end the call, change the effects, then start a new call. Also, trying to switch Skype from the CamTwist back to the default Logitech webcam while CamTwist is running causes Skype to 'quit unexpectedly'. My other problem is that everything comes out reversed, so text displays backwards.
On the whole very pleased with it despite the glitches, and it's a bargain given that it's free. It's worth a donation, but I see there have been no updates for nearly a year so am not sure if the developer is still around?