It just works! Specifically, keeping the system running and up-to-date is very low-maintenance. On the software side, the pkg system has been great—and a welcome change from manually installing ports from source. On the OS side, security updates and upgrading major and minor versions have all been painless.
I haven't run a Linux server long-term, so I can't compare, but my biased perception from the outside is that it seems like Linux changes more rapidly and requires a bit more hands-on management.