It really depends on how you use your Mac. If you are careful and don’t download and install software from questionable sources and are aware of warnings that Safari gives, I don’t think you need antivirus for now.
Besides being careful with what you download and install, turn your firewall on (In System Preferences/Security), and consider not using the administrator account for day to day work. And be sure Software Update is set for automatic under Software Update under the Apple Menu.
Traditional, old style viruses from the Windows world can’t infect Macs, but some of the newer malware is written in a language called Java that can run on both Macs and Windows and as a result can effect Macs. (And the newer malware isn’t technically a virus, either; it’s a trojan horse program).
But there is still a big difference between Windows and Mac malware, and especially this most recent stuff. The most recent Java-based malware that is circulating must be installed for it to infect your Mac. That means you must allow the malware to install itself by giving the Java applet permission to run in Safari. Many times on Windows, malware can infect without you doing anything or even knowing that it has happened, and Java-based malware is the newest vector against Windows.
This is also the case of other malware over the past years that affects Macs; it all must be installed by either allowing it in Safari or by you typing in your password to install it as a program. So if you’re careful and aware of where your programs come from and deny programs and applets that Safari warns you about, I think you’re safe. And if you do your day to day work in an account that doesn’t have administrator privileges, the malware can’t install, anyway.
Antivirus programs for Macs can be buggy, they can slow down your Mac, and sometimes introduce security problems of their own. But one of the cases where having antivirus on your Mac will help is if you work with people who use Windows. Antivirus will warn you if you are about to pass a Windows virus on to others. Those viruses can’t infect your Mac, but you can pass them on to Windows PCs.
The latest version of OS X (Snow Leopard) has some basic malware protection. It’s not updated very often, and for now, it only detects a few kinds of malware, and it doesn’t detect this latest malware. Apple may expand this built-in malware protection in the future. See Mac OS X – Keeps safe from viruses and malware. Apple has indicated recently that it may stop using Java in future versions of OS X, anyway, and that will prevent all Java-based malware from being able to work.
But because Apple’s market share is increasing, I think OS X will become a target of more malware, either based on Java or other exploits that haven’t been discovered yet. Tech writers and security experts have been predicting the end of OS X’s near-malware free state for years. It all depends on market share, they say. Windows has 80-90% of the market, and that’s where the malware is right now, because that’s where the money is for the criminal element behind malware.
And if you want to run antivirus anyway (which will protect you and also help prevent passing on viruses to Windows users, too), you can try the free and reputable Sophos Antivirus for Mac. See https://www.sophos.com/en-us/products/free-tools/sophos-antivirus-for-mac-home-edition.aspx