has anyone tried icinga or opennms, and can comment on that ?
It's also the opposite of nagios in that rather than lots of smaller moving parts, it is one big mega (java) process that does everything. Again, not necessarily a problem (though I happen to think so :), but different.
I also found opennms to be VERY complicated. I suppose nagios is though, first time around.
For some reason though, I really want to use opennms and keep going back to try it out, but eventually give up.
Your traditional monitoring systems have hand-selected features to monitor and alert for. OpenNMS will just go out and discover everything you have (and graph everything without any intervention too).
You probably aren't monitoring all the statistics on every interface of your switches (what? people have switches?), but just throw OpenNMS at your networking management subnet and it'll pick up everything for later review.
You can use OpenNMS for alerting and inventory tracking, but I prefer more extensible tools for those. Just use OpenNMS as a largely hands-off sanity check of your existing monitoring and graphing systems.