I just bought a Logitech QuickCam VC for my iMac (333Mhz, 96MB), and it's causing major crashes. When I open the application software, either QuickPict or Quick Movie, I get a moving image for a few seconds. Then a message flashes up: "Camera not available." Then the screen freezes and I have to force-quit. Often I have to force-quit the finder too, and sometimes even have to get the paperclip out.
Has anyone else had these problems? Is it Mac's ongoing difficulties with USB? Or Logitech's poor programming? I'm using QT 4.21 – is this another QT4 streaming problem?

I think your problem is USB. I have a QuickCam VC on my iMac (series A, OS 8.5.1), and had to add all the firmware and USB patches to make it work right (but its fine now). Also, plug it directly into the iMac, not via the keyboard, as it doesn't get enough power otherwise. I don't think you have a QT4 problem here. I would have expected the new iMacs to have all the patches, but your problems sound exactly like mine.