We started experimenting with Nagios six years ago to get a feel of it as a recommended network monitoring system. We tested other products like opennms, zabbix and zenoss but we finally decided to go for Nagios as it was very extensible. At that time, only Nagios could be configured to work with our in-house developed SMS-based messaging system. This is probably the greatest advantage of Nagios - it can be customized to a degree to suit your monitoring needs. It's architecture also allows distributed monitoring, which is really a great feature to reduce network traffic.
Room for Improvement:
Nagios' great customizability is also one of it's greatest drawback. In the early days, installation and configuration of nagios was not for the faint of heart. You need a lot patience and system administration skill to figure out what gets into what file. This is still true today, as some still install nagios from source. Aside from installation, configuring nagios from the command line is very tedious, time consuming and prone to user errors. The core nagios installation from source does not provide an integrated management system for nagios, and you have to install these systems separately from nagios.Nagios is a great product. I would highly recommend this to organizations which requires a great deal of flexibility in terms of customizing their network monitoring system.
Though the core nagios system is still very challenging to install, a lot of bundled installers with very good GUI's for configuring nagios now exists. Instead of doing everything manually from the command line, you can just grab one of these packaged forks and get started with nagios in as little as 10 minutes.