Their claim that they will replace your Mac if it gets a virus is false advertisement. Apple is hiding the truth about infection. While it may be true that Macs don't get virus (as yet), they do get infected by other forms of malware like spyware. So if you get infected by one of those, if you go to an Apple store and complain, they will say "Oh, we were talking about virus, not spyware so your case doesn't apply."
Would Norton, Kaspersky, BitDefender, Avast, ClamAV, Sophos and other big and reputable anti-virus companies sell products for Macs if there's no need for it? Do you know how many man-hours they've invested to create an 'anti-virus'? I would like to hear Mac users answer this.
Apple Mac OS X Server has included ClamAV since version 10.4. Isn't that a clear indication that Macs need protection against infections?
That said, I read that ClamAV and Sophos Anti-Virus are good. Visit their websites and read their forums and see what infected Mac users say.