OS X Mavericks is the biggest thing to happen to OS X since the debut of OS X. I've been using it as my primary OS for the past week and even though it is still in beta, it is totally stable and performs extremely well.
If OS X Lion and Mountain Lion seemed to be incremental and updated for the sake of updating, OS X Mavericks 10.9 is THE update to get! Everything just flows together, works well and has been tweaked and efficiency has been pushed to the max. Less thrashing of the hard drive, Compressed Memory, App Nap, Safari and Finder Tabs are the highlights of this operating system. Everything has been fine tuned to such an extent that it just seems perfect. (I'd been always wondering why Apple doesn't put the unused apps to sleep to save RAM and energy, and Apple has done it! I also really wished for a bigger iPhone after squinting to read on my iPhone and Apple did that too, and made the iPad....Apple seems to have habit of thinking before most people do and provide them the facilities...Maybe that's why there are so many iSheep...me included :) )
It is equivalent or even more monumental than Windows XP or 7 were to the Windows operating system. It doesn't seem like it on the surface, but it is a landmark operating system. If you have been holding off upgrading till now (and like me downgraded back to Snow Leopard for it's efficiency), it's finally the right time to upgrade....