I was a long time Nagios user but the manual config changes and management of it was just getting too much, I've recently switched to running a clustered OpsView setup monitoring 2 geographically separated sites and around 1000 devices. It "just works". It is easy to configure and manage, the data warehousing/searching/reporting feature is great, graphing is excellent, dashboard and nagvis elements let you present data nicely and there's even a scheduled reporting tool to email the management a PDF full of pretty graphs every month. Because it's built on Nagios there's a plugin for monitoring just about anything you can think of and it's free! I don't work for OpsView, I'm just a fan!