Is MacOS 10.6.8 Apple's XP?
It's reliable, runs well on older hardware and a smaller memory footprint, and doesn't have much bloat by today's standards, yet performs all the necessary functions of an OS. It's also well understood by power users and seems more configurable than newer versions.
Even though 10.9 is free, I rarely see it installed on machines capable of running it at the university I work at. I considered installing it on my white plastic MacBook, but decided against it after a terrible experience with 10.7 (incessant VM paging vs. 10.6.8 in 2GB RAM and still pretty bad in 4GB) and downgraded. Is 10.9 any better in this regard?