Seconded. I used it at the previous job and we were most satisfied with it (although the web front end's CGIs being not easily modified compiled C rather than perl could be considered a strike against :).

Thirded, sort of. This is one of those I know a bloke who knows a bloke whose sister used.. kind of endorsements :-) A sysadmin I used to know used netsaint back in the day and used to rave about it constantly. netsaint then morphed into Nagios (as I recall) and I've heard that it's only got better since it's netsaint days.