I downloaded OS 10.9 'Mavericks' as soon as it appeared in the Mac App store on Wednesday morning, NZ time. This was onto my own MacBook Pro, my partner's 2010 27-inch iMac and onto the new 2013 Haswell iMac I still had as a test unit, with its Fusion Drive. It took me about 30-40 minutes to download in each case.
Immediately after restart, I ran the Mac App store app again and downloaded other updates that appeared, notably iPhoto and iTunes, restated again, ran Disk Utility>Repair Permissions (always a good idea after a system update - it's an Apple app in your Utilities folder) and restarted again. I noticed some cosmetic changes immediately - a menu on each of my two screens, for example, that stays semi-transparent on the screen you aren't using. Yay! I love this and I've wanted it for years - until now, you had to set which screen had your Menu Bar in System Preferences>Displays. Under Arrangement, where you dragged the white menu bar to the screen you wanted as your primary, and that's the only one that could have a menubar.
Also, now when you Full Screen something on one, you can still use the other, so I can watch a movie in iTunes on one screen while I ahem ... work on the other. (I know, bad habit.) But the price is you can no longer stretch apps across two displays.
Also, my Dock had two new items - Maps, which shows great detail, although doesn't go in as close as Google Earth and doesn't have Street View, but it was accurate enough to place my in the right part of my house, which was a bit freaky. (WiFi needs to be turned on for this to work.) It also has a pretty cool-looking 3D view, plus Traffic and Directions buttons. I'll probably use this a lot, as I like to see where I'm going for a general idea before trusting to the TomTom app on my iPhone while in transit. Directions are shown as blue lines, while traffic buildup is shown as red lines and less problematic but building-up traffic as orange dots.
The other new item is iBooks, which updates any iBooks you have in iTunes (which used to only work on your synced iDevices). I had 71, as I'm a fan of this genre. Indeed, I've even published one, called 'What Did You Do During The War?' about car companies and wartime production during the last century.
I know, riveting, right?