Its got a nice front end and it seems to be well built and it also can use ANY nagios plugin you install.
[link|http://www.zenoss.com/download/links|http://www.zenoss.com/download/links]
The Virtual Appliance actually WORKS. I tested it a bit ago, locally, but not time to play right now.
Nagios works, its just a PITA to configure. Defining hosts, so that DNS can fail and everything still works. Using naming conventions to keep the names short on pages to phones.
I know, 1200 services monitored (adding 300 this week or so), 300 interfaces to keep track of, some are additional IPs on interfaces... but I still gotta whack them every three minutes. Plus twigging router with SNMPD, Hosts with SNMPD, out of order checking on SNMPGET SUCKS and is tough to track down.