What do you like best about the product?

Nagios, for me, is the swiss army knife of monitoring. The design of nagios monitoring is a breath of fresh air. There are other monitoring systems where you can monitor equipment in ways the original author of the software thought you should do it. Still, because of nagios' design to use command-line oriented interfaces to monitoring, you can touch anything and get valuable information from it, and parse it into graphs, etc with standard plugins.

What do you dislike about the product?

I dislike the steep learning curve. I've been using Nagios for some time, so it's relatively easy for me, but I have colleagues who have never used systems like nagios before and they have a hard time getting up to speed with it. Related tools such as NagioSQL can really help to get you going with usable configs, but ultimately you need something like that or your own scripts that you may create, to add services & hosts.

What problems is the product solving and how is that benefiting you?

We use nagios for solving problems with internal and external systems that need guaranteed uptime. In our infrastructure, there are internal & external systems, and we use nagios to gather status and performance data so that we know if/when things happen. The fact that nagios is able to send us alerts via sms or email, or anything else that can be addressed from a command line interface, is absolutely awesome. Once colleagues get familiar with the configuration, or once they learn the rules of the plugin environment, they can start setting up monitoring for services which are actually composite-services. For example, it's one thing to monitor if a mail server is up and running, it's quite another to know that the mail users are getting their mail in a timely fashion from the outside. Compositing services together and monitoring them from nagios makes it possible to get an idea of "this is what the end user really experiences" rather than just raw numbers.

Recommendations to others considering the product:

It has a bit of a learning curve. I would recommend using a tool such as NagioSQL to create config files. The config files are definitely not meant to be created by a human, they should be machine generated because of the complexity and dependencies involved.