best for a mac, and it's popular enough that drivers have been written for it for windows, though the mic support is lacking. i have a high end logitech (640x480@30fps, CCD). it works fine in OS X for video and pics, but doesn't have driver support for video chatting and such and it doesn't work w/ the iLife suite (in iMovie u can do direct captures via the iSight, kinda like a firewire camcorder). from what i've seen at the apple store the iSight also has better image quality than my logitech.
also, firewire has lower CPU overhead than USB2, which means your apps can do the work they're trying to do while the cam does what it's supposed to do.