Nagios XI Review

We use Nagios because it does exactly what we need: Alert us when something is wrong and (with PNP) show some trend to figure out when or maybe how the problem started. The graphical output is for sure not state of the art, but we are not artists but rather techs ;) As a hosting business we have quite a homogenous infrastructure and use mostly snmp for fetching data in our own scripts. Once the basic setup for Nagios is done it is quite easy to automatically script new hosts when you add new machines or delete the old ones. However your choice might be different if you have to monitor IT of various customers with totally different infrastructure.
And finally there are apps for android, iPhone, etc so with just one look at your phone you see if everything is well.