😮😟
Yes, I'm actually shocked and sad because in 2 weeks, I didn't get one single response. I am positive I am not the only person who has an iMac and who wants to use another webcam aside from the built-in iSight. Anyway, I guess if you want something done right, you better do it yoursef. For those of you who may at some point have a similar question, I hope this helps.
I wanted a webcam that could be mounted on a mini tripod so that it didn't have to stay static on top of the screen but could be moved and placed in a variety of places without worrying about leveling the cam, the cam falling, etc (look at the Joby Gorillapod Original). Anyway, after doing as much research as I could stomach, I ended up with two main choices. A Logitech C615 and a Logitech C920. They're both basically the same except that Logitech calls them "better" and "best." Unless the incredible definition of the C920 is essential to you, the C615 will do the job quite well. I ended up buying the C920 because it came up on sale. Alas, Apple rules when it comes to ease of use. I cannot download the software from Logitech for all the special effects and stuff, but the camera (and stereo microphone) are plug-and-play. YES, THE C920 WORKS WITH OS X MOUNTAIN LION. I can say without doubt that the C615 will work too. Let's just say that the picture provided by the camera is incredible. I recorded a 30 second movie under exactly the same condtions on Quicktime with the built-in cam and with the C920 and there is no comparison, the C920 blows the iSight away.
So far, I have tested every single application I could think of and every single one of them allows me to pick between the built in iSight or the Logitech cam, my iMac recognizes the cam automatically. The only one that doesn't seem to detect it is iMovie, but that may be only due to my lack of familiarity with that app. So, Skype, Facetime, Quicktime, PhotoBooth, Messages ALL work with the Logitech C920 AND OS X Mountain Lion 10.8.2
A word of advice. If you're like me and you're not too concerned about the extreme high definition, in retrospect I would buy the C615. Why? Simply because it can also be mounted on a mini-tripod, stand alone, or bracketed on top of your screen, BUT also because it doesn't only swivel up and down but also sideways (the C920 only swivels up and down. No huge drawback, but we're talking getting the most for your money). The C615 is also about $30 to $40 CDN cheaper.
Anyway, sorry about the long post. I hope this helps other people and I save them the time, effort, and disappointment of asking a question and having nobody answer it 😟