Before we get into the specifics of the product, it's important to know what were actually trying to protect ourselves against. Firstly, there are viruses - bits of software that are secretly introduced to your computer, carry out some sort of unauthorised activity and spread themselves to other computers secretly.
Then there's malware. This is subtly different to a virus in that it relies on you installing it - usually without your knowledge or by tricking you. For example, when Apple and Adobe released some new software recently, malware developers introduced some malicious code into pirated versions of the software.
Phishing scams rely on fooling you into surrendering information such as website log-ins, credit card details or personal information.
Security conferences and experts have shown proof-of-concept Mac viruses but we haven't been able to find a real, in the wild, virus that affects OS X reported anywhere. However, malware infections and phishing scams are a real threat. That's why, when you take a look at the shelves of your local reseller you'll see that there's very little "anti-virus" software - it's all about the security software.
Older Mac versions of Norton's software were pilloried for the way they slowed a computer down. The old joke was that the reason the picture of Peter Norton on the software packaging had him posed with a stern look and crossed arms was that he was waiting for the software to work. Many Macs were slowed to a crawl as the application chewed up all the CPU cycles and memory it could find.
Fortunately, Symantec totally rewrote the Norton code, starting from scratch. It's now far leaner.