Malleability is Nagios' true strongpoint. I have seen it used in 3 fairly different environments. I've used 3 different frontends, currently using 2, to interact with the nagios backbone. Each having their own strengths and weaknesses respectively. When it comes to interacting directly with it via the CLI, while not the nicest to look at or work with, it does exactly what you tell it to.

It monitors everything we threw at it. It can even tie into pagers (yes, old style beepers) to deliver notifications. Down time and maintenance notifications are easy to use. The add ons really make this product shine -- with the help of free libraries of addons and plugins online there is very little that can't be monitored by this software. Plus, the price is right!