Sorry to start yet another thread, but after reading some negative comments, I decided to chime in. I have upgraded my 2011 MBP and MBA. The MBP reported a couple of incompatible extensions but no other problems. No problems on the MBA. I am having fun checking out all the new features and launching all the apps. Both computers are running great. Noticeable improvement in speed, feel, fun. Best 20 bucks I ever spent.