The ease of use, scalability, availability of plugins for other 3rd party products are some of the best features that I like about this product. It is open source so there can be many customizations done to suit your needs. So if you do not find what you need in the default configuration then it's easy enough to customize as per requirements. We have used OpenNMS to monitor our Cisco switches and routers. This has really helped us in troubleshooting issues with the devices very quickly. And also we were able to create alerts when certain events occurred which otherwise would've been difficult to detect without using an alerting system.