This guide does a pretty good job of describing the situation:
http://guides.macrumors.com/Mac_Virus/Malware_FAQ
Bottom line: There are a few bits of malware out there, but by and large,
OS X's built-in defenses do a pretty good job of protecting against them. For anything new, I highly doubt most AV products will catch up any sooner than Apple's own mechanisms would.
The few things out there that might be a threat are those which trick the un-savvy user into escalating admin privileges to install them. At that point though, even antivirus software won't protect you. If the user is handing over the keys to the castle, there's not much else that can be done, really.
So, unless you KNOW you're going to be dealing with lots of potentially virus-laden files, it's probably not worth it. Most AV programs are resource hogs, and the performance hit you take from running them is often not worth what little protection they provide.
If you do want to install Antivirus software though, I'd recommend
ClamXAV. It's open source, and useful for scanning suspicious files you might encounter. And it runs when
you tell it to, so the resource cost is minimal.
As an aside:
Definitely avoid installing snake oil software that claims to "improve performance" on your Mac.
Mac Keeper is the most controversial of these. At best, they do nothing you could've done yourself for free. At worst, some of them may actually install adware, spyware or malware.
Memory cleaners are also something you should be suspect about, as OS X (starting with Mavericks) does its own pretty good RAM management. And the "full memory" these programs claim to clear out is really the point of that memory management: OS X fills up your RAM with caching and other useful data to make the system run faster... making you get your money's worth out of your memory. Actively clearing that out all the time only works against the OS.