FCS is pretty good, if you are looking for a video-conferencing solution that doesn't entail the user downloading special software. But there are lots and lots of little headaches on the authoring side -- inconsistent implementation of the event model is my buggaboo, lately. And if you write your own server scripts, it can be a huge task keeping your client-request/client-response code straight. If you just use the components, it is about as easy as cutting and pasting.
If you have a remote FCS server, there's not much of a difference between Mac and Windows. At the moment, you can only run the server on RedHat Linux or Windows and I don't think there are any plans for the Mac. I have heard of folks getting it working on Gentoo, but I strongly doubt you'll get the server to work on Mac.
The only problem I've run into is that the Logitech webcam I use seems to get 'associated' with the first application that uses it. That means if you preview your fcs movie in Flash MX, you can pretty much forget viewing it in a browser or the stand-alone player until you *shut down* Flash MX (and I'm not talking about viewing the camera in Flash and a browser at the same time). I have no idea if this is the case for other people.
If you want to find out more, there is an FCS listserv (email discussion group) over at
http://chattyfig.figleaf.com/mailman/listinfo/flashcomm . You can pretty much ask any question there and someone will answer much better than I could.