I have used Ubuntu, Kubuntu, and Edubuntu with Wiziq. I also have Xubuntu on my netbook now, Lubuntu is not worth it and I recommend people stay away from it. The main thing in Kubuntu was that Asian fonts were not supported, however, I could still copy and paste into the chat room and whiteboard without problems. I just couldn't see it there, but I could see it in the area I copied from.
This is real close to perfect for me. I use other programs in Windows which are not as useable in Linux, so I still toggle between the two.
As for screen sharing, I don't understand why people would need Wiziq for that. I understand Skype has it, and here are a few others http://www.makeuseof.com/tag/7-easy-screen-sharing-and-remote-access-tools-all-free/
I never got into the screen sharing stuff anyway. For me, I think they should focus on delivering audio, pictures, and text on all platforms possible before embarking on video elements.