I was recently asked my opinion on Anti-Virus software for the Macintosh.  Here’s my reply:

Most Mac folks will say “none.”

I sometimes use Intego VirusBarrier (VB), but there are others, some free, most paid.
 
Leaving aside the whole issue of “is it really necessary on Macs” one thing to consider when looking at a package is what other features it offers.
 
Most AV stuff will examine files already on your drive (takes forever) and incoming files as well.

Intego VB also includes a firewall, trojan protection (that’s what MacDefender/Protector et al is); and other attacks; protection against phishing, cookie filters; ad banner filtering; information hiding and other web threats. It will look for keyloggers and hacking tools, corrupted resource forks, and problmes with iPhone/iPod/iPad software and suspicious memory corruption. There’s included Geek stuff too, like Whois, packet tracing, network discovery, and traffic analysis. Even a cute little meter to show you incoming and outgoing traffic.
 
That’s a huge list of stuff, making VB pretty much a one-stop-shop for all kinds of stuff that can be done individually with other (multiple) software packages.
 
Like all such software, it requires a subscription to keep the known problem files up to date. And like all such software, it cannot protect against “zero-day” attackes. (That is, no software can protect you against something completely new and different, since by definition, it’s new and unknown. So prompt updating of the security files is important. Fortunately, it’s also automatic, if you set it up properly.)
 
Do I use it?  Oh gadz…  sometimes I have it installed, and sometimes I uninstall it.  (Contrary to what you may commonly hear, I’ve seen no significant deterioration of CPU performance with it in use.)
 
At this point, it has saved my bacon twice. Once a few years back when what was purported to be a Flash installer instead turned out to be a bit of trojan malware. And a day or so ago. At the time I actually got the .dmg MacProtector file, VB is -not- installed, and I caught the file myself (by virtue of keeping up with the news, and hence recognizing the name.)  As a test, before I deleted the malware, I installed VB again and then copied the malware to my hard drive. VB literally honked up a huge warning virtually instantly… so had it been installed at the time of the actual download, it would have caught it right away.
 
As far as my own anecdotal use of VB goes, there is no downside that I’ve seen, other than sometimes when I want to connect to a service that VB doesn’t know about, it will prevent me from doing so until I tell it that it’s OK. I’ve not noticed any “drag on the system” for using it (other than when it does a full-drive scan, over which you have control.)
I kinda like having the granular control I have over individual IPs and logs and files and basically everything internet/LAN. Equally, that’s -way- too geeky to appeal to most casual users. Fortunately, there’s a set-it-and-forget-it simple mode as well.
 
As far as software AV packages go, I’m pleased with all the features it offers, and its one-stop-shop approach.
It has worked, performing its functions as advertised in the past.
 
What’s more, I’m actually behind a hardware firewall, -and- on NAT, so I don’t really need the firewall features, but they are useful for folks not using my kind of gear.
 
So… should you use it?
 
As far as the protection from malware goes, one could rationally argue either way, since there is -almost- no malware for Macs that don’t involve you specifically granting it permission to install or run. (“Almost” is the operative word here, since there -are- sites you can visit that will allow someone access to your Mac without you doing anything except visiting the site.)
 
If you do not have a firewall, and are directly attached to the internet via your “modem” then the firewall features are nice to have. That said, the OS has a firewall built in, and it’s free. All you have to do is turn it on.
 
So… leaving aside the fact that it costs money, the question becomes “is there any reason to NOT use AV software?”
 
Given that it 1) doesn’t slow down the computer; 2) provides a firewall; 3) will warn you about phishing and malware, 4) relieves you of some anxiety; my answer would be “No – there’s no reason to avoid using it, other than the cost.”
 
Finally, am I running VB right now?  Yes. Given that I was shot in a drive-by, I’ll leave it on for a while.  Why not?