I have been using Nagios for infrastructure monitoring across mixed Windows and Linux environments, and it feels like a rock-solid workhorse rather than a flashy modern SaaS. The biggest strength is its plugin ecosystem and flexibility: almost anything with an IP address can be monitored if you are willing to invest time in configuration. The web UI and initial setup can be intimidating, especially compared to newer tools, and maintaining complex configs requires discipline, but once tuned, alerts are reliable and performance is stable even under heavy load. For teams that value control, extensibility, and on-prem monitoring over ease-of-use out of the box, Nagios remains a very capable and trustworthy choice.