We are actively testing Mountain Lion ourselves. We’ve noticed a few small things, like an issue with EnVideo or EnVE. But so far nothing major.
If you are using EnSight 10 it should be fine. 10.0.2(d) is what we’ve tested here in Marketing. Displaying EnSight onto our screen using an Apple TV and Airplay was simple and fun.
If you are using EnSight 9, then be aware that X11 is no longer bundled with the Mac OS 10.8. You will have to download it from Apple or the internet if you want to use EnSight 9.x or earlier on Mac OS 10.8.