I'm running NIS 2009 on 3 computers at the moment with zero problems and a minimal footprint in terms of resources. Tests run by CNET indicate that the latest version of Norton Antivirus runs as fast or faster on a number of tasks including boot time in comparison to the competition.
Symantec has received a lot of bitter well deserved criticism over the years but to their credit they have listened and corrected the resource hog problem and the intrusive popups problem. I have been running NIS since 2006 with no problems.
Norton's strength and weakness is the same: it's a big company. At times it has been slow to correct problems and the software has been bloated; however, with a big company you also have a lot of people working on product development and especially advanced malware protection. Currently the protectiveness of their product is generally excellent and definitions are updated every 4 minutes to make sure that you always stay ahead of the bad guys.
Of course with all the money being spent by criminals to come up with more sophisticated attacks nothing is certain but there is an argument to be made for having a company with lots of resources to work on new strategies to counter new threats.
I've tried McAffee and Zone Alarm in the past, the first had a lousy (unintegrated) interface whereas the second was buggy; that was years ago though so they've almost certainly improved.
One problem with Norton is that if you have a problem it's pretty hard to get it resolved; you're usually better off with googling error messages or problem descriptions than calling tech support, if that doesn't work you may end up having to switch to a different product.
