It is easy to create your own custom modules if you just know a little bit of scripting. If you have unique requirements, you can just make your own modules.
You can even grab checks from other vendors. There are open-source checks for various things such as SMTP, etc. There is a long list of different ones from Nagios. You can just use them, and within seconds, you get yourself a check that is monitoring whatever you need. It is really flexible. I guess that's why they call it Pandora Flexible Monitoring System (FMS).
It is reliable. It does the job, and it alerts. It is also surprisingly feature-rich. Our network guy just recently asked about a particular protocol to monitor the bandwidth on the network, which is not a common protocol. When I looked it up, and I found that they cover it. It is very mature for a not-so-known product.
