Whats the best Network Management software (NMS)?

Hi guys,
Im a 3rd year degree student and for my thesis (dissertation) I’m investigating whether open source NMS software is suitable for most networks, or better than proprietary alternatives…

I have so far studied exactly what a NMS is, and what is expected of a good NMS.

I have downloaded and used SpiceWorks which seems great, but wondered if any body out there has used any other NMS’s (open source or proprietary) and are willing to give me a brief review of your experiences?

I am most interested in hearing from people who have used openNMS, Nagios, Zenoss and other multi-platform solutions. I am also interested to find reviews on proprietary solutions as it seems this area is dominated by open source software.