My personal experience is that Nagios is probably the LEAST easy to use of any piece of software, period.
You've obviously never installed/configured RT3.
I used to use Big Brother, tried Zenoss after reading about it, then tried Nagios. Nagios totally rocks.
It's not THAT hard to configure. We use Nagios, Snort, Ntop and RT3. Hard to say how much money I saved by not using any of the big commercial products, but it's a lot. But RT3, crap, THAT is hard to get going..