I have used Norton AV for over one year at this point and have not had any issues. That's not to say it is great Mac software, but it does it appear to do its job effectively. Quirks include:
-- an all but meaningless/useless icon wasting space on the top bar;
-- Live Update window is graphically interesting but is forced on top of all other active windows while it is open (you can move it out of the way, but that gets old on a daily basis);
-- very slow connection speeds to Norton server for updates;
-- in general, a barely Mac-like interface...; and
-- expensive to initially purchase.
One major positive though, the annual renewal for virus defs just cost me only $3.95. No new complete version to wonder about (which is not needed for the Mac platform anyway, i.e. what's a new scanning engine going to offer a machine that barely needs one in the first place). I used to update the whole software/engine/version every year during my Windows days out of fear of missing something. So, while you pay more at the beginning (arguably like a Mac), you start saving money by the second year of use.
It really boils down to 'do you want to protect Windows users from themselves?' I use Office 2004 on my PB, and I share files with co-workers/office server/work laptop, and so on. So, in the end, while I barely need protection on my personal machine, I am relatively confident I have done what I can to protect myself and others. Peace of mind, and part of the solution rather than the problem.
The real question, I think, is the firewall, particularly with a cable ISP...another story for another day...