Last Tuesday Apple officially launched OS X 10.9 Mavericks. Unlike previous versions of OS X, this update is available for free via the Mac App Store for all Mac users.
OS X used to be a premium upgrade (like Windows), however thanks to Apple's vertical business model they are able to subsidise the cost of software with the sale of hardware. This is an issue for other companies such as Microsoft, as their hardware footprint is small and therefore they must rely on software licensing to make a profit.
The other reason this approach makes sense for Apple is that OS X updates have become increasingly evolutionary over the past few releases, in fact you could argue that OS X has not seen a major update since 2007 with the release of Leopard. However, with a "free to upgrade" policy for all users, this lack of innovation is made easier to accept.
As a result Mavericks is a subtle upgrade over Mountain Lion, brining a number of "under the hood" features that focus on improving battery life, performance and security. The user interface itself remains mostly unchanged (see my minimal desktop below):
One thing that Apple has done with Mavericks is reverse direction on the use of skeuomorphic design, for example removing the linen stitching from Mission Control and the Notification Centre. This delivers a cleaner, simpler user interface that I personally believe is a significant improvement. The screenshot below shows Mission Control and it's texture-less background.
The same "flat" design philosophy has been followed for the built-in OS X applications. For example, Calendar no longer has the unnecessary leather user interface, replaced with the classic OS X grey. In my opinion these subtle changes help unify the operating system, delivering a consistent look and feel.
Unfortunately the team working on Game Center must have missed the memo as it's the one application that retains its skeuomorphic design. As a result it sticks out like a sore thumb and in my opinion is a stark reminder of what a miss-step this design approach was. Thankfully I (and apparently Apple) rarely use Game Center and therefore it does not negatively impact the overall experience.
The only major user facing feature upgrades are part of Finder, which now finally supports tabs, removing the need for third party applications such as Path Finder. Finder also supports tagging for all folders and files, which improves search and enables a more versatile way of managing your file system. Both features are a welcome addition, but otherwise Finder usage remains unchanged from Mountain Lion.
Finally, it's worth mentioning that not only is Mavericks more power efficient at its core, but it also allows the user to better understand application energy usage. For example, Activity Monitor has been redesigned to include an "Energy" tab which allows the user to see which applications are negatively impacting battery life. This is great news for all users as Mac notebooks are rapidly moving towards the vision of "all day usage on battery".
Overall OS X Mavericks is a solid update! It feels fast (booting and shutting down in seconds), reliable and appears to be compatible with all existing applications. Even historically troublesome applications such as VMware Fusion (5/6) work fine and you will likely find your Mac App Store applications have already been updated to leverage the new core capabilities of Mavericks. As a result I would not have any issue recommending Mavericks to all Mac users, especially as it's a free upgrade.