I've been using Virus Barrier for 3 or 4 years. Easy to use and fairly complete but after working with technical support we could not find a solution to my Mac sometimes freezing after waking from sleep, when Time Machine was backing up, or when surfing. Lots of "virusbarrier blocked application for 60 seconds" with app after app listed until my MacBook finally hung.
Tried ClamXav and no problems with it hanging. Scans take a long time, and it flags a lot of emails if they are sent to one email address I use, but not another. Everything from my Mileage Plus card is flagged as a virus (background scanner) or phishing (full scan) even though it's a statement with my account balance.
NIS is about 1/2 the size of Intego's internet suite. Scans quickly and with the default settings is unobtrusive. I did not give it 5 stars due to it's clunky interface. Most things are fairly straight forward but for others such as setting up a custom firewall based on location it is not. The help menu is basic and does not cover everything. Going to the online help section usually takes you to prior Mac versions or the PC version. Still trying to figure out if the virus database updates happen automatically, or if I have to manually do them. There seems to be no way to schedule when to do the update. You cannot specify when to do a full scan, but you can set it to do so when your Mac is inactive.