Good flexibility in terms of rules configuration, which gives you many options to adapt itself according your requirements (you can define custom actions for all kinds of triggers from simple services restarting or CPU utilization exceeding over 30%). For every check there are different types defined by its type-of action taken - notify for example could be one notification only while service down state might have another kind of notifications with email sending etc., so it definitely makes easy decision about what will happen when certain condition takes place on monitored systems. Easy setup but requires some initial knowledge how system like icinga works before applying them. You basically need knowledge where do those checks come into play (listen localhost interface?). In my case our production website was hosted somewhere else than server monitoring box with IP address visible to outside world.
Checkbox didn't worked since no connection between web page served through apache/nginx webserver could send alert message directly using standard port 80 traffic via internet without involving dedicated servers (we needed real time feedback). Some times even though we see alert notification nothing happens if checked rule has been already set to trigger, but next second everything starts working normally again after timeout configured and then alerts start triggering as expected! So please test first! If yes use this product at least couple weeks until something new comes up and maybe The most helpful feature about icinga for me was its ability to monitor multiple servers in one dashboard. It's also very easy to setup, configure and use. A lot of features are disabled by default which can be quite frustrating at times but it's not hard to turn them back on if you need or want them. We were able to get all our server information into one place instead of having several different dashboards open.