We used OpManager in production for over a year. It has terrible Linux support. None of their built-in plugins worked properly for monitoring even basic parameters like disk space, free memory, CPU usage, etc. When we pointed this out to their support people, they said we should build our own plugins with SNMP OIDs. Um....no. Not for the amount of money we paid for that steaming POS. We finally kicked OpManager to the curb about a month ago, and have our entire environment, Windows and Linux servers being monitored with Nagios. Nagios scales well, we are currently watching several hundred hosts and about 3500 services.
OpenNMS is also a good tool, its ability to map servers back to switch ports is extremely handy.