I managed to get my hands on a C920 because I was curious as to how well it worked with Macs. Here's what I've found so far:
1. In Message, when I click preferences and the Audio/Video tab, the C920 immediately becomes active. The spectrum analyzer that lights up with the mic detects audio is also active. Everything seems to be working perfectly.
* None of my friends use video chat so I haven't be able to test it in an actual video chat.
2. In QuickTime, I was able to do a quick little recording. Video and audio both worked perfectly.
3. Photo Booth seemed to work perfectly.
All of this was done using just the built-in OS X drivers. No additional software was installed. Once I plugged the C920 in to an available USB port, it started working right away.
The image quality, in my opinion, is far superior to my old original Firewire iSight. The image (again, in my opinion) is sharper, less grainy, and brighter. There is no discernible difference in audio quality between the C920 and Firewire iSight, but, admittedly, I have never been confused with an audiophile.
I'm going to hang on to the C920 for a few days before returning it to my buddy. Feel free to ask me to test for compatibility, I'll try to help where I can.