In 25 years of being a Macintosh computer user, I have had one machine infected one time, and that was due to my own foolishness when I was a relatively young and new computer user.
I won't rehash all of the old arguments regarding the pros and cons of having an antivirus app installed on one's hard drive. That issue has been dealt with more than enough.
My main point -- and I am not dissing this app in particular -- is that I don't see the necessity or the logic of spending so much money on an antivirus app, when simple and free ClamXav seems to do the job just fine.
Now, if you happen to work with a lot of Windows machines and Windows files, maybe you need and require more robust protection. However, for the average Macintosh user, I think that freeby antivirus apps like ClamXav are sufficient.
It really comes down to adhering to good, safe computing practices, plain and simple.