I was going to suggest ipMonitor (which Solarwinds acquired a few years ago). They just changed the licensing so it's $1000-$2000 for an unlimited number of monitors. The system has a basic mapping engine, NOC environment, sound/email/pager/exe/etc alarms, complex SNMP, WMI, and SQL alarms for everything fmor basic up/down to user experience types of alarms, etc. I never had to look into distributed pollers and I don't think it supports that. The biggest con I ran into was that the reporting engine seemed slow and there was no way to get at the raw data to export it out to other tools.
Other pros include wizards (enter an IP to scan and it will suggest a list of monitors for that device by scanning for SNMP, WMI, etc), ability to create dependancy chains so that you would receive alerts from 5 devices when their shared switch to the backbone went down (you would just receive the ones for the switch), ability to create "Smart" groups which were basically dynamic groups that include all devices/monitors that meet a set of search criteria, etc.
Not sure how it scales, but we had this running against 100+ servers and network switches from a little virtual server w/ maybe 1GB of RAM and it didn't seem to be hurting. We also used Cacti for another perspective into traffic flows an such to give us another dimesnion to use when troubleshooting (monitoring + flows + logs).