Mountain Lion hit the Mac App Store yesterday afternoon, and I was one of those people who upgraded within like three hours. Not the smartest move historically (I've learned that lesson before), but curiosity won out. I'm writing this on 10.8.0, so take everything here with the understanding that it's still the bleeding-edge version with all the bugs that implies.
First, the notifications system is everywhere. Apple's clearly looking at iOS and thinking "people kind of like knowing when stuff happens," and now Notification Center is sitting in the top-right corner of your screen watching everything. It's not bad, actually. Mail notifications, Messages notifications, all stacked up in a place that doesn't immediately distract you the way Growl notifications did. But I've already got too many apps trying to notify me about things I don't care about. Twitter, I'm looking at you.
Messages is the other major addition. iMessage integration, proper syncing with your other devices—it's the logical move for Apple. SMS and iMessage all in one place. The thing that's bugging me though is that it feels like Apple's trying to build their own Gchat alternative but only if you already own an Apple device. Which, sure, their users mostly do, but my sister's on Android and my best friend refuses to upgrade from a 3GS, so the unified messaging thing doesn't really work for me yet. It'll probably get better. It's day one.
The tweaky stuff is more annoying right now. Some older apps are already throwing up compatibility warnings. Sparrow (which I use for email because Gmail's web interface makes me want to flip my desk) is fine, but I've got a few utility apps that are clearly gonna need updates. The usual startup disk space concerns apply too; Mountain Lion's sitting there taking up all the room while I delete old stuff.
Gatekeeper is actually kind of interesting if you care about security. App Store apps plus trusted developers only, if you want that setting. It's not the most elegant solution, but it's something. Makes me wonder how long before someone figures out how to get around it, but at least Apple's trying to give people an option between "trust anything" and "only Apple."
Power Nap is there too—your Mac keeps doing stuff when you think it's sleeping, updating Mail and Calendar and things. Useful? Maybe. Battery drain issue waiting to happen? Possibly. It's turned on by default, which feels a little presumptuous, but I get it.
The whole thing feels iterative rather than revolutionary. Not that I expected otherwise: this has been the Mac strategy for a while now. Take the popular iOS stuff, figure out how it works on a laptop, ship it. Mountain Lion's just another step in that direction. It's not a bad step; it's just not the kind of thing that makes you go "oh wow, I had no idea I needed this." Except maybe the notifications thing, which I'll probably actually use.
Performance-wise it seems fine. Doesn't feel slower or faster than Lion was. Opens apps at the same speed. If there's overhead happening, I can't feel it yet.
What's bugging me most is that I know I'm gonna find random broken stuff over the next few weeks. That one app that nobody uses but you love will stop working. Some shell script you rely on will silently fail because of API changes. That's just the upgrade tax. I've paid it enough times that I should've waited until at least 10.8.1, but no, I had to poke the bear immediately. At least I backed everything up first, which is more than I can say for myself two years ago.
Anyway, if you're on Snow Leopard or Lion and you're wondering if you should jump in, maybe wait a week or two. See what the weird bugs are, let the dev community find the scary stuff. Me, I'm already in it, so I might as well write about what I find.