I installed Mountain Lion on all three of my Macs as soon as it came out. Just to check it out and see what it would do to my systems performance and the general usability of the systems.
What I can say from these couple of days of usage is that Mountain Lion is obviously the OS X version that Lion was supposed to be. It is fast, it is polished, it has a couple of new neat features and even though I was worried, it didn't turn my MAC into another iPad - the down-dumbing of the OS is by far not as bad as expected.
The changes and additional features are more or less evolutionary and just make the MAC better. I have installed it on a one year old MacBook Air 11", a two year old iMAC 27" and a two year old MacBook Pro - and as I said, the update went fine and the performance is good. Curiously the update on my MacBook Pro took much longer than on the other two systems - but afterwards everything worked just as well.
Everything looks just a little bit mor polished than before even though it is hard to say why. I like the new notification system which works well for me. Airplay is just a wonderful feature long overdue on the MAC. Besides that the new Reminder and Notes Apps help me to be a little more organized - now I have these across all my Systems including the iPad and iPhone. iMessage was already out for quite a while - so I am already used to it. iCloud integration only gets better (I still keep my Dropbox though - interoperability with my Windows and Android systems is still important to me).
I have to say - very nice update for just 20$ for all of my systems. The only App I am missing is iBooks for OS X - but I guess there must be something left for the next update.