Open source and very flexible. large ecosystem of tools and custom monitors built up around nagios. All configuration is text files, so it is easy to keep this in version control and generate new configs from other tools.
Room for Improvement:
Default interface is clunky and slow. Can be a steep learning curve if you haven't worked with it before. Adding devices or services requires reloading the config or restarting the service. Would like for this to be more dynamic. It seems that most of the new development is going towards Nagios XI(paid enterprise version) rather than the opensource nagios. Would like to see some of the newer ideas in Icinga and/or MK livestaus integrated into opensource nagios.
Disclosure: I am a real user, and this review is based on my own experience and opinions.
Sep 04 2012