Prior to discovering Zenoss I had been a long time Nagios user. I had dabbled in the other major open source players in this arena as well, including Big Brother, Munin, Hyperic and Groundwork.
After 4 years of writing Nagios customizations I was a little disappointed to learn that Zenoss already did everything I had spent so much time developing, and MUCH more straight out of the box.
To top all of this off, Zenoss has the backing of a real company. Not only does this provide the obvious advantage of having a support channel standing behind the product, but more importantly it allows for a very aggressive release schedule. I've lost count of the number of times I asked, or even thought of a feature I would love to see only to see it exist in the latest release.
If you're looking for an all-in-one network & systems management platform with event management and a configuration database, this just could be the only valid option out there unless you have the bucks to have HP or IBM come in a develop something for you.