Is it generally advisable to get anti-virus software for Mac OS X? Is it worth to buy an anti-virus software for Mac OS X?

I heard both opinions like "it's not possible to get a virus on Mac OS X" and "Mac OS X is also vulnerable against virus... "

Which is true? What is the accepted practice, if any? Under what circumstances would anti-virus be advisable?
There is anti-virus software available for Mac OS X:


Intego VirusBarrier
ClamXAV (Free)
Norton AnitVirus Mac Edition
McAfee VirusScan for Mac
Sophos Anti-Virus for Mac (Consumer Version is free)


Is it worth it? That's largely up to you. Personally I'd say unless required there's no reason to so long as you practice "safe computing" (eg. not opening files from just anywhere, being aware of what is installed on your computer, etc.). However there are some companies that require any computer (regardless of OS) to have up to date anti-virus software installed and operational.

There are no viruses available for Mac OS X but there are several pieces of malware and trojan horses that a user must manually download and install. The anti-virus products (and actually a feature in Mac OS X Snow Leopard) will help fight against these threats but the primary defence is actually user education as the attack vector is primarily social engineering and not using any security hole.