I've recently upgraded my machine to 10.8, and I've been using FCP 6 on it. No issues that I've noticed so far, but I also am not dealing with any 3rd party plugins or hardware, so my setup may not be an accurate reflection of most production environments.
If you can, install 10.8 (now at 10.8.1) on a spare machine (or spare drive) and test your setup when you can between projects.