I'm really happy to read from more than one developer/tester that Mountain Lion is way faster and zippy than Lion. Someone even writes that it's like Snow Leopard.
I was really afraid that with all that iOS controlled features, the new OS would have slowed down even more the machine, but it looks like this won't be the case!
Fingers crossed.
In total, Mountain Lion is a good upgrade. I don't need any of that useless Facebook/Twitter/Social Network crap that crawled into the OS, but it has MANY little improvements and features that make Mountain Lion a much better operating system with higher usability than Lion or any other of its predecessors. After having used it for a coupe of days, I am not sure whether it is really faster than Lion in everyday use; after a fresh install it felt faster, but once all my other software was on the system, I didn't feel that much of a difference anymore.
That brings me to this rather important point:
Do NOT make an upgrade installation! Wipe the hard disk and install a FRESH version of Mountain Lion. I tried a direct upgrade from Lion and afterwards the system was slow and behaved erratic to a point that made it unusable. A clean installation fixed all this for me.
One other thing: If you have a Cyborg RAT7 Contagion mouse, forget about it. The mouse is not yet compatible with Mountain Lion and it causes a ton of weird behavior of the dock, the menu bar and even your web browsers. The current available driver version is 1.1.42 and it does NOT solve any problems on Mountain Lion. Wait for the next release.
By the way, Microsoft Office 2008 and even Adobe Photoshop CS3 work both well under Mountain Lion. The only problem with CS3 is that it no longer connects to Adobe's update servers, you have to download and fetch the available updates manually.
Growl from the Mac AppStore also still works properly and runs side by side Apple's new notification system. Personally, Apple's notification system is of no use to me - I don't use Safari or any other of Apple's mini applications that come with the operating system, so I have yet to see the first notification coming through that pipeline. I'm sure that will change over time, but at the moment it doesn't do anything for me.
Gatekeeper: I prefer to call it Dungeon Keeper, but it doesn't really matter because it was the first feature that I had to turn off - VLC does NOT run if you have Gatekeeper activated, and VLC is my default video player.
Apple has a major design flaw in Gatekeeper - you CANNOT leave it turned on and simply add exception for that one application that does not run when Gatekeeper is turned on. It's either all or nothing. So I'd say that this whole security design is useless and Apple does not deserve any laurels for it. But then again, I still suspect that they did not care for security at all and only tried to push more users into the Mac AppStore.