Mountain Lion seems very Promising, It runs just as smooth or even smoother then Lion which is great since Lion ran perfectly on my MBP. The Airplay mirroring is by far the best improvement to this OS and it seems like every day i find new features that weren't available in Lion. The only issues that i have with this update are the changes that they made to "Preview" since i am a grad student and i use this program to take notes on the PDF notes they provide in class. Also the icons in launchpad seem to be a little bigger then in Lion and thus a little blurry (barely noticeable but its there). Due to the bigger Icons in launchpad the folders hold less apps in each one. Notification is a great feature that i think should have been in Lion. Safari 6 is a huge improvement as well! this is my first Mac that i have had for about six months so i got used to the single bar in chrome. I am very happy they combined the HTML and Search bar in Safari 6. Every app that worked in Lion seems to work just as well in ML! As for Growl, Notification did not make it obsolete since Notification only works with apps that are downloaded through the app store. so anything installed from outside the app store growl will still be need. Other then the changed to Preview i believe ML is worth the $20!