Zabbix has been part of my toolbox for quite some time. I can easily say it's an indispensable tool for me now.
Managing a dozen servers without Zabbix would be unimaginable. I'm monitoring all of this: CPU, Memory, Hard-drives, website response times, downtime. The UI might be a bit "old school", but everything works flawlessly.
With regards to hard-drive monitoring, I love the machine learning option that allows you to "predict" the number of days before running out of space. That's quite helpful, as I've got some of my servers down due to running out of space multiple times in the past (before I was using Zabbix).