Good security software does not just think of viruses
Advanced Windows security software usually includes other features besides (live-) scanning the computer for infections. They also provide protection from:
- Phishing attacks (attempts to steal your passwords)
- Spyware (exploiting private information)
OSX maintains a list of malicious software which you can update automatically (System Preferences → Security & Privacy → General).
If you want advanced protection by a specialized company, I'd say go for it. But if you use your computer sensibly and browse the web carefully, there is no need.
The renown german IT website Heise says that Apple can take months to fix a known security vulnerability. Microsoft on the other hand, takes threat reports very seriously. They encourage communication with the 'community' in providing security patches. "Replies from Apple are not to be taken for granted," they say.
Third-party security companies generally provide protection from newly discovered threats quite fast.
Working in a Windows environment
At the university I exchange files via flash drives or emails on a regular basis with Windows users. Some of those flash drives have been exchanged between lots of users which led me to use antivirus software myself in order to minimize the possibility of transmitting threats.
I've been using Sophos Mac Home (free) for a long time. Sophos updates it's signature list frequently. While the live-scan does not seem to add any significant CPU load most of the time, there are cases where it impacts the use quite noticeably. I'm thinking of repairing permissions lasting more than ten minutes instead of just one or spotlight indexing.
1 - Heise reported that this list contains only a relatively little amont of signatures and a smart attacker can easily bypass it. As of now, this security features is rather to calm down the user than offering advanced protection. Their words are that "any virusprotection software provides better a better solution".