OpsView is basicly on web frontend for Nagios. It delivers GUI for both monitoring and configuration.
One of the best things about it is the integration of various 3rd party applications. It has MRTG, NagVis, NMis, NagiosGraph, SMS senders, NDO and much more. The installation may not be the easiest but after you finish it - it works! Some extra configuration may be required for extra modules (like for NagVis).
Also - because it uses NAgios as the monitoring engine - you can add all the cool stuff you could add to a standard Nagios installation. And its fast.
Another upshot is the implemented distributed monitoring solution. Adding slave monitoring nodes is not a problem here.
It also has its drawbacks. It has no integrated application for NetFlow monitoring. There is a documentation for installing and configuring FLAVIO to work with OpsView. But FLAVIO (from what I can gather) is pretty much dead.... Another downside is lack of batch host configuration. Every single host hast to entered manually via GUI. There is the "clone" option but there should be a "mass add" script (maybe there is but I could not find one).
All in all this is a great piece of software. I would recommend it to all who want the power of Nagios but don't have the time/will/knowledge to tinker with it. It is also a great teaching platform for learning the basics of Nagios mechanics.