I like that it's easy to configure rules in order to monitor my network devices (servers/applications).
It also has an API which makes integration with other tools very simple as well! There are some features missing such as support of SNMP v3 or OID resolution but this could be solved by adding more developers into its team so they can add these new functionalities themselves. We use wazer mainly because we want our servers monitored 24x7 without having too many alerts generated each day from different sources.