After hearing not so bad reviews, I took the plunge and upgraded to mountain lion. I do have some questions after the ugrade: Do I have to have calendar, chat etc physically open, even if minimized for the iMessage to work? I had to add more gmail and yahoo accounts, so temperature information that I get on my ipod calendar automatically synced with the mac the first time but not today. Also, are there advanced settings for the iMessage app in Settings other than the appearance and beeping options for the apps listed in Settings? There was no way to add any other apps to the list already population in settings. I guess iOS would have to go from 5 to 6 before the icloud feature of synced tabs would work with my Mac? Overall, there isn't a whole lot of difference only that I should be able to install this on multiple computers and the fact that somehow I shaved off 6 GB off of my storage after installing ML which is COOL !