Mac OS X is a series of UNIX-based operating system developed by Apple Inc. OS X is developed only for Apple Mac systems.
Mac OS X 10.9 is most used version of OS X with 2.79 percent of market share in OS arena.You can say its bad because of its low market share.
In fact OS X is the most advanced OS in the world. The reason for low market share is that OS X is only designed for Mac computers and Mac are quite expensive.
OS X is most beautifully crafted OS among all.