Lots of questions!
NetView will monitor the snmp status of your devices as long as it.
a) Has the read community string
b) The MIB for the device is loaded (a lot are loaded by default)
If a device goes off the network you will know, and if an interface in a router goes off (assuming it can still comunicate to NetView), you will know.
NetView can receive snmp traps and display an alert based on this (ie high CPU, no memory etc).
NetView can be used to collect snmp traffic (ie interface stats) and can graph them.
NetView will integrate into most hardware vendor specific management tools, ie ciscoworks, transcend or optivity.
I would recommend getting NetView 7.1.3 (FP01) as version 7.1 was a little unstable.
There is a lot more that you can do with NetView (smartsets, web console, nvsniffer for port monitoring), it's just a case of reading the manuals and redbooks (and have at LEAST 512MB Ram). The hardest work is setting up the netmon.seed file and the location.conf. Get these right and it will discover the right devices quickly and display the network in a way that makes sense.
Hope this helps