Today is the day. For Mac users looking to upgrade, or who have done so already, I'd recommend glancing at this list:

http://www.apple.com/osx/whats-new/features.html

There are tons of little fixups and tweaks compared to Lion, some easy to miss. Still working my way through the John Siracusa epic review (http://arstechnica.com/apple/2012/07/os-x-10-8/).

I've been running the GM on my machines pretty much since it came out, and I've not been hit by any regressions from Lion. With the release today also comes the iWork update for iCloud. This weekend I'll spend some time moving my old iDisk documents into iCloud and decide if this is what I want. If it doesn't work out, I'll look into an iDisk replacement.

Gatekeeper so far hasn't been a problem, and I plan on keeping it in it's default setting of "Mac App Store and identified developers". When you do an upgrade install, any existing unsigned app is still allowed to run. Any new unsigned apps will be blocked via double click, but can be allowed by simply context clicking and choosing open. Once you open it once, the system stops blocking it.

Not really sure on the tabs thing Bruno, perhaps a hidden preference will be revealed now that the OS is officially out. For history, there is always the History - Show all History menu item. The unified bar hasn't bothered me yet, since it seems to still do the right thing and visit internal servers when I type their names. Chrome in the same situation enjoyed tossing me to a google search, and usually required a manual http:// to get it to go to internal servers.