Yes, we are using it. It is pretty good. Of course as a developer whos applications are being monitored by it I'm a little biased. It is pretty feature rich however. We have it monitoring various things like:
Simply establish a connection to the database.
Run a query against the database and analyze the results.
Monitor some Microsoft MSMQs. If the number of messages in the queue is greater than say 5,000, raise an alarm.
Monitor the memory usage of a service. It it exceeds a certain amount, restart the service.
Monitor some log files and check for the occurance of certain text.
All alarms are sent as SMS text messages to our cell phones, but you can also do a bunch of other stuff.
The administration is all web based.
Hope that helps.