This caused flame war in my Mac users group, but the recent advisory that Apple released and then pulled suggesting using anti-virus products. My take is that it's worth being safe rather than sorry, what's the general feeling on serverfault?
And if yes what do you recommend?
If you share files with windows users at all, I'm sure they would appreciate you doing your part to sanitize any files you give them first.
Thinking only about whether or not you and your machine will be affected could lead to you becoming a carrier, unknowingly bringing malware into your office network - especially if it is a laptop (or a machine that is used both inside and outside the network.)
If it's a corporate environment, then yes you should be using anti-virus - it's a general liability and butt covering thing (if you operate a mixed network it's also prudent - no point in having your Mac users forwarding macro virus laden emails and the like to your Windows/Linux users)
If it's your home machine, then you might look at exactly the same question from the PC side: Do you use an anti-virus application on your home computer?
In general Macs are not heavily targeted, and much like a PC, common sense is one of the best anti-virus products available. The vast, vast majority of PC and Mac "viruses" are not clever hacks or attacks on operating system vulnerabilities, they are social engineering tricks. As a Mac user the single most common "virus" you will encounter is a "pirated" copy of Office/Photoshop/etc that is only 3MB in size and is asking for your admin password to install.
Viruses are harder to get on macs, but that doesn't mean they can't get them. It really depends on how you use your computer. If you are smart and don't open links which you don't know where they come from, and visit sites which you shouldn't, and don't download things like warez, you'll probably be ok. That being said, it's always easy to make a mistake. It wouldn't hurt to run it, and it's always better to be safe than sorry.
I've been using Intego VirusBarrier X5 for about a year and in that time I've seen it catch something it didn't like on one occasion. So even though viruses on Macs are a rare occurrence, they do happen. Also points above (@David, @Brent) about multi-platform environments are well made. The Intego product also traps common Windows threats. I can heartily recommend it...
AV software isn't without it's performance costs as well as it's monetary costs. You have to balance the likelyhood of infection (or worse, you infecting someone else) against the performance hit that you'll take by running it. You might, for instance, decide to run AV software on a regular scan, but not run an 'on-the-fly' scanner that intercepts every file open.
If you are networked with a lot of other computers or regularly send/receive files from other users, you REALLY should have something (at the least, you don't want to accidentally be a vector for a windows virus). If you don't connect to other computers, and don't do much file transfer, you can probably afford to be more lax.
Actually, the apple products are less affected by virus/spyware but that only because the market is much more smaller than the PC market.
People say that MAC is much more secure than Windows. I'm not agree with this. MAC also have is flaws, that's only because hacker/virus creator (and the other) aren't interest to it because, as I said, the market is much smaller and they want to hit big, so they target PC.
That said, it doesn't mean you don't need an AntiVirus or any other protection. YOU SHOULD have it, because there a tendency for more virus for mac as the mac market slowly grow.